services: configuration: Document multiple configurations per field.

* gnu/services/configuration.scm (generate-documentation): Filter for all
configurations associated with each field-name.
* doc/guix.texi (Complex Configurations): Describe use case for multiple
field-names.

Change-Id: I6d1e7b8858231aa05fda884b31a9bd0a48f07293
Signed-off-by: Ludovic Courtès <ludo@gnu.org>

doc/guix.texi

@@ -44702,7 +44702,9 @@ available for the configuration record.
 @var{sub-documentation} is a @code{(@var{field-name}
 @var{configuration-name})} tuple.  @var{field-name} is the name of the
 field which takes another configuration record as its value, and
-@var{configuration-name} is the name of that configuration record.
+@var{configuration-name} is the name of that configuration record.  The
+same value may be used for multiple @var{field-name}s, in case a field
+accepts different types of configurations.
 
 @var{sub-documentation} is only needed if there are nested configuration
 records.  For example, the @code{getmail-configuration} record

gnu/services/configuration.scm

@@ -7,6 +7,7 @@
 ;;; Copyright © 2021 Andrew Tropin <andrew@trop.in>
 ;;; Copyright © 2022 Maxime Devos <maximedevos@telenet.be>
 ;;; Copyright © 2023 Bruno Victal <mirai@makinata.eu>
+;;; Copyright © 2024 Herman Rimm <herman@rimm.ee>
 ;;;
 ;;; This file is part of GNU Guix.
 ;;;
@@ -460,8 +461,12 @@ DEFAULT."
                            (para ,@field-docs)
                            ,@(append-map
                               generate
-                              (or (assq-ref sub-documentation field-name)
-                                  '())))))
+                              (filter-map
+                                (match-lambda
+                                  ((name config)
+                                   (and (eq? name field-name)
+                                        config)))
+                                sub-documentation)))))
                fields)))))))
   (stexi->texi `(*fragment* . ,(generate documentation-name))))