doc: Rewrite define-configuration.

Rewrite this section to make it easier to document later syntactical
changes.

* doc/guix.texi (Complex Configurations): Rewrite define-configuration
documentation. Fix simple serializer example.

Signed-off-by: Maxim Cournoyer <maxim.cournoyer@gmail.com>

doc/guix.texi

@@ -42576,54 +42576,33 @@ and to make it easier to create Scheme bindings for these configuration
 files, you can use the utilities defined in the @code{(gnu services
 configuration)} module.
 
-The main utility is the @code{define-configuration} macro, which you
-will use to define a Scheme record type (@pxref{Record Overview,,,
-guile, GNU Guile Reference Manual}).  The Scheme record will be
-serialized to a configuration file by using @dfn{serializers}, which are
-procedures that take some kind of Scheme value and returns a
-G-expression (@pxref{G-Expressions}), which should, once serialized to
-the disk, return a string.  More details are listed below.
+The main utility is the @code{define-configuration} macro, a helper
+used to define a Scheme record type (@pxref{Record Overview,,,
+guile, GNU Guile Reference Manual}).  The fields from this Scheme record
+can be serialized using @dfn{serializers}, which are procedures that take
+some kind of Scheme value and translates them into another Scheme value or
+@ref{G-Expressions}.
 
 @defmac define-configuration name clause1 clause2 @dots{}
 Create a record type named @code{@var{name}} that contains the
 fields found in the clauses.
 
-A clause can have one of the following forms:
+A clause has the following form:
 
 @example
 (@var{field-name}
- (@var{type} @var{default-value})
- @var{documentation})
-
-(@var{field-name}
- (@var{type} @var{default-value})
+ @var{type-decl}
  @var{documentation}
- (serializer @var{serializer}))
-
-(@var{field-name}
- (@var{type})
- @var{documentation})
-
-(@var{field-name}
- (@var{type})
- @var{documentation}
- (serializer @var{serializer}))
-
-(@var{field-name}
- (@var{type})
- @var{documentation}
- (sanitizer @var{sanitizer})
-
-(@var{field-name}
- (@var{type})
- @var{documentation}
- (sanitizer @var{sanitizer})
- (serializer @var{serializer}))
+ @var{option*}
+ @dots{})
 @end example
 
 @var{field-name} is an identifier that denotes the name of the field in
 the generated record.
 
+@var{type-decl} is either @code{@var{type}} for fields that require a
+value to be set or @code{(@var{type} @var{default})} otherwise.
+
 @var{type} is the type of the value corresponding to @var{field-name};
 since Guile is untyped, a predicate
 procedure---@code{@var{type}?}---will be called on the value
@@ -42641,6 +42620,28 @@ an object of the record type.
 @var{documentation} is a string formatted with Texinfo syntax which
 should provide a description of what setting this field does.
 
+@var{option*} is one of the following subclauses:
+
+@table @asis
+@item @code{empty-serializer}
+Exclude this field from serialization.
+
+@item @code{(serializer @var{serializer})}
+@var{serializer} is the name of a procedure which takes two arguments,
+the first is the name of the field, and the second is the value
+corresponding to the field.  The procedure should return a string or
+@ref{G-Expressions} that represents the content that will be serialized
+to the configuration file.  If none is specified, a procedure of the
+name @code{serialize-@var{type}} will be used.
+
+An example of a simple serializer procedure:
+@lisp
+(define (serialize-boolean field-name value)
+  (let ((value (if value "true" "false")))
+    #~(string-append '#$field-name " = " #$value)))
+@end lisp
+
+@item @code{(sanitizer @var{sanitizer})}
 @var{sanitizer} is a procedure which takes one argument,
 a user-supplied value, and returns a ``sanitized'' value for the field.
 If no sanitizer is specified, a default sanitizer is used, which raises
@@ -42654,21 +42655,7 @@ symbols looks like this:
         ((symbol? value) (symbol->string value))
         (else (error "bad value"))))
 @end lisp
-
-@var{serializer} is the name of a procedure which takes two arguments,
-the first is the name of the field, and the second is the value
-corresponding to the field.  The procedure should return a string or
-G-expression (@pxref{G-Expressions}) that represents the content that
-will be serialized to the configuration file.  If none is specified, a
-procedure of the name @code{serialize-@var{type}} will be used.
-
-A simple serializer procedure could look like this:
-
-@lisp
-(define (serialize-boolean field-name value)
-  (let ((value (if value "true" "false")))
-    #~(string-append #$field-name #$value)))
-@end lisp
+@end table
 
 In some cases multiple different configuration records might be defined
 in the same file, but their serializers for the same type might have to
@@ -42689,13 +42676,13 @@ manually specify a custom @var{serializer} for every field.
 
 (define-configuration foo-configuration
   (label
-   (string)
+   string
    "The name of label.")
   (prefix foo-))
 
 (define-configuration bar-configuration
   (ip-address
-   (string)
+   string
    "The IPv4 address for this device.")
   (prefix bar-))
 @end lisp
@@ -42787,11 +42774,6 @@ Return a G-expression that contains the values corresponding to the
 disk by using something like @code{mixed-text-file}.
 @end deffn
 
-@deffn {Procedure} empty-serializer field-name value
-A serializer that just returns an empty string.  The
-@code{serialize-package} procedure is an alias for this.
-@end deffn
-
 Once you have defined a configuration record, you will most likely also
 want to document it so that other people know to use it.  To help with
 that, there are two procedures, both of which are documented below.
@@ -42894,7 +42876,7 @@ Below is an example of a record type created using
 
 (define-configuration contact-configuration
   (name
-   (string)
+   string
    "The name of the contact."
    serialize-contact-name)
   (phone-number
@@ -42904,15 +42886,15 @@ Below is an example of a record type created using
    maybe-string
    "The person's email address.")
   (married?
-   (boolean)
+   boolean
    "Whether the person is married."))
 
 (define-configuration contacts-list-configuration
   (name
-   (string)
+   string
    "The name of the owner of this contact list.")
   (email
-   (string)
+   string
    "The owner's email address.")
   (contacts
    (list-of-contact-configurations '())