services: Add descriptions.

* gnu/services/dbus.scm (polkit-service-type)[description]: New field.
* gnu/services/dict.scm (dicod-service-type)[description]: New field.
* gnu/services/dns.scm (knot-service-type)[description]: New field.
* gnu/services/networking.scm (dhcpd-service-type)[description]: New field.
* gnu/services/shepherd.scm (shepherd-root-service-type)[description]:
New field.
* gnu/services/xorg.scm (slim-service-type)[description]: New field.
(screen-locker-service-type)[description]: New field.
* gnu/system/pam.scm (pam-root-service-type)[description]: New field.
* gnu/system/shadow.scm (account-service-type)[description]: New field.

gnu/services/dbus.scm

@@ -1,5 +1,5 @@
 ;;; GNU Guix --- Functional package management for GNU
-;;; Copyright © 2013, 2014, 2015, 2016, 2017, 2019 Ludovic Courtès <ludo@gnu.org>
+;;; Copyright © 2013, 2014, 2015, 2016, 2017, 2019, 2020 Ludovic Courtès <ludo@gnu.org>
 ;;; Copyright © 2015 Sou Bunnbu <iyzsong@gmail.com>
 ;;;
 ;;; This file is part of GNU Guix.
@@ -363,7 +363,13 @@ (define polkit-service-type
                             (append (polkit-configuration-actions config)
                                     actions)))))
 
-                (default-value (polkit-configuration))))
+                (default-value (polkit-configuration))
+                (description
+                 "Run the
+@uref{http://www.freedesktop.org/wiki/Software/polkit/, Polkit privilege
+management service}, which allows system administrators to grant access to
+privileged operations in a structured way.  Polkit is a requirement for most
+desktop environments, such as GNOME.")))
 
 (define* (polkit-service #:key (polkit polkit))
   "Return a service that runs the

gnu/services/dict.scm

@@ -1,6 +1,6 @@
 ;;; GNU Guix --- Functional package management for GNU
 ;;; Copyright © 2016 Sou Bunnbu <iyzsong@gmail.com>
-;;; Copyright © 2016, 2017, 2018 Ludovic Courtès <ludo@gnu.org>
+;;; Copyright © 2016, 2017, 2018, 2020 Ludovic Courtès <ludo@gnu.org>
 ;;; Copyright © 2017 Huang Ying <huang.ying.caritas@gmail.com>
 ;;;
 ;;; This file is part of GNU Guix.
@@ -174,7 +174,12 @@ (define dicod-service-type
                              (const %dicod-activation))
           (service-extension shepherd-root-service-type
                              dicod-shepherd-service)))
-   (default-value (dicod-configuration))))
+   (default-value (dicod-configuration))
+   (description
+    "Run @command{dicod}, the dictionary server of
+@uref{https://www.gnu.org/software/dico, GNU Dico}.  @command{dicod}
+implements the standard DICT protocol supported by clients such as
+@command{dico} and GNOME Dictionary.")))
 
 (define* (dicod-service #:key (config (dicod-configuration)))
   "Return a service that runs the @command{dicod} daemon, an implementation

gnu/services/dns.scm

@@ -639,7 +639,10 @@ (define knot-service-type
                         (service-extension activation-service-type
                                            knot-activation)
                         (service-extension account-service-type
-                                           (const %knot-accounts))))))
+                                           (const %knot-accounts))))
+                (description
+                 "Run @uref{https://www.knot-dns.cz/, Knot}, an authoritative
+name server for the @acronym{DNS, Domain Name System}.")))
 
 
 ;;;

gnu/services/networking.scm

@@ -335,7 +335,9 @@ (define dhcpd-service-type
    (name 'dhcpd)
    (extensions
     (list (service-extension shepherd-root-service-type dhcpd-shepherd-service)
-          (service-extension activation-service-type dhcpd-activation)))))
+          (service-extension activation-service-type dhcpd-activation)))
+   (description "Run a DHCP (Dynamic Host Configuration Protocol) daemon.  The
+daemon is responsible for allocating IP addresses to its client.")))
 
 
 ;;;

gnu/services/shepherd.scm

@@ -105,7 +105,11 @@ (define shepherd-root-service-type
    (extensions (list (service-extension boot-service-type
                                         shepherd-boot-gexp)
                      (service-extension profile-service-type
-                                        (const (list shepherd)))))))
+                                        (const (list shepherd)))))
+   (description
+    "Run the GNU Shepherd as PID 1---i.e., the operating system's first
+process.  The Shepherd takes care of managing services such as daemons by
+ensuring they are started and stopped in the right order.")))
 
 (define %shepherd-root-service
   ;; The root shepherd service, aka. PID 1.  Its parameter is a list of

gnu/services/xorg.scm

@@ -1,6 +1,6 @@
 ;;; GNU Guix --- Functional package management for GNU
 ;;; Copyright © 2017 Andy Wingo <wingo@igalia.com>
-;;; Copyright © 2013, 2014, 2015, 2016, 2017, 2019 Ludovic Courtès <ludo@gnu.org>
+;;; Copyright © 2013, 2014, 2015, 2016, 2017, 2019, 2020 Ludovic Courtès <ludo@gnu.org>
 ;;; Copyright © 2015 Sou Bunnbu <iyzsong@gmail.com>
 ;;; Copyright © 2018, 2019 Timothy Sample <samplet@ngyro.com>
 ;;; Copyright © 2019 Jan (janneke) Nieuwenhuizen <janneke@gnu.org>
@@ -619,7 +619,9 @@ (define slim-service-type
                          (service-extension profile-service-type
                                             (const (list xterm)))))
 
-                  (default-value (slim-configuration)))))
+                  (default-value (slim-configuration))
+                  (description
+                   "Run the SLiM graphical login manager for X11."))))
 
 (define-deprecated (slim-service #:key (slim slim)
                                  (allow-empty-passwords? #t) auto-login?
@@ -691,7 +693,11 @@ (define screen-locker-service-type
                  (list (service-extension pam-root-service-type
                                           screen-locker-pam-services)
                        (service-extension setuid-program-service-type
-                                          screen-locker-setuid-programs)))))
+                                          screen-locker-setuid-programs)))
+                (description
+                 "Allow the given program to be used as a screen locker for
+the graphical server by making it setuid-root, so it can authenticate users,
+and by creating a PAM service for it.")))
 
 (define* (screen-locker-service package
                                 #:optional

gnu/system/pam.scm

@@ -1,5 +1,5 @@
 ;;; GNU Guix --- Functional package management for GNU
-;;; Copyright © 2013, 2014, 2015, 2016, 2017, 2019 Ludovic Courtès <ludo@gnu.org>
+;;; Copyright © 2013, 2014, 2015, 2016, 2017, 2019, 2020 Ludovic Courtès <ludo@gnu.org>
 ;;;
 ;;; This file is part of GNU Guix.
 ;;;
@@ -367,7 +367,13 @@ (define pam-root-service-type
 
                 ;; Arguments include <pam-service> as well as procedures.
                 (compose concatenate)
-                (extend extend-configuration)))
+                (extend extend-configuration)
+                (description
+                 "Configure the Pluggable Authentication Modules (PAM) for all
+the specified @dfn{PAM services}.  Each PAM service corresponds to a program,
+such as @command{login} or @command{sshd}, and specifies for instance how the
+program may authenticate users or what it should do when opening a new
+session.")))
 
 (define* (pam-root-service base #:key (transform identity))
   "The \"root\" PAM service, which collects <pam-service> instance and turns

gnu/system/shadow.scm

@@ -378,7 +378,10 @@ (define account-service-type
                        (service-extension user-processes-service-type
                                           (const '(user-homes)))
                        (service-extension etc-service-type
-                                          etc-files)))))
+                                          etc-files)))
+                (description
+                 "Ensure the specified user accounts and groups exist, as well
+as each account home directory.")))
 
 (define (account-service accounts+groups skeletons)
   "Return a <service> that takes care of user accounts and user groups, with