services: Add 'description' fields.

* gnu/services/admin.scm (rottlog-service-type)
(tailon-service-type): Add 'description' field.
* gnu/services/audio.scm (mpd-service-type): Likewise.
* gnu/services/avahi.scm (avahi-service-type): Likewise.
* gnu/services/ssh.scm (lsh-service-type)
(openssh-service-type, dropbear-service-type): Likewise.

gnu/services/admin.scm

@@ -183,6 +183,9 @@ for ROTATION."
 (define rottlog-service-type
   (service-type
    (name 'rottlog)
+   (description
+    "Periodically rotate log files using GNU@tie{}Rottlog and GNU@tie{}mcron.
+Old log files are removed or compressed according to the configuration.")
    (extensions (list (service-extension etc-service-type rottlog-etc)
                      (service-extension mcron-service-type
                                         rottlog-jobs-or-default)
@@ -323,6 +326,9 @@ for ROTATION."
 (define tailon-service-type
   (service-type
    (name 'tailon)
+   (description
+    "Run Tailon, a Web application for monitoring, viewing, and searching log
+files.")
    (extensions
     (list (service-extension shepherd-root-service-type
                              tailon-shepherd-service)

gnu/services/audio.scm

@@ -97,6 +97,7 @@
 (define mpd-service-type
   (service-type
    (name 'mpd)
+   (description "Run the Music Player Daemon (MPD).")
    (extensions
     (list (service-extension shepherd-root-service-type
                              (compose list mpd-shepherd-service))

gnu/services/avahi.scm

@@ -1,5 +1,5 @@
 ;;; GNU Guix --- Functional package management for GNU
-;;; Copyright © 2014, 2015, 2016 Ludovic Courtès <ludo@gnu.org>
+;;; Copyright © 2014, 2015, 2016, 2017 Ludovic Courtès <ludo@gnu.org>
 ;;;
 ;;; This file is part of GNU Guix.
 ;;;
@@ -117,6 +117,11 @@
 (define avahi-service-type
   (let ((avahi-package (compose list avahi-configuration-avahi)))
     (service-type (name 'avahi)
+                  (description
+                   "Run @command{avahi-daemon}, a host and service discovery
+daemon that implements the multicast DNS (mDNS) and DNS service
+discovery (DNS-SD) protocols.  Additionally, extend the C library's name
+service switch (NSS) with support for @code{.local} host name resolution.")
                   (extensions
                    (list (service-extension shepherd-root-service-type
                                             avahi-shepherd-service)

gnu/services/ssh.scm

@@ -186,6 +186,9 @@
 
 (define lsh-service-type
   (service-type (name 'lsh)
+                (description
+                 "Run the GNU@tie{}lsh secure shell (SSH) daemon,
+@command{lshd}.")
                 (extensions
                  (list (service-extension shepherd-root-service-type
                                           lsh-shepherd-service)
@@ -460,6 +463,8 @@ of user-name/file-like tuples."
 
 (define openssh-service-type
   (service-type (name 'openssh)
+                (description
+                 "Run the OpenSSH secure shell (SSH) server, @command{sshd}.")
                 (extensions
                  (list (service-extension shepherd-root-service-type
                                           openssh-shepherd-service)
@@ -543,6 +548,8 @@ of user-name/file-like tuples."
 
 (define dropbear-service-type
   (service-type (name 'dropbear)
+                (description
+                 "Run the Dropbear secure shell (SSH) server.")
                 (extensions
                  (list (service-extension shepherd-root-service-type
                                           dropbear-shepherd-service)