docs/devel/qapi-code-gen: Update cross-reference syntax

The new QAPI code generator creates a cross-reference target for each
definition documentation.  Enabled for the QEMU QMP Reference manual
in commit a377f39f38, and for the QEMU Storage Daemon QMP Reference
Manual and the QEMU Guest Agent Protocol Reference in commit
a6af544344.  We've put these targets to use since, but neglected to
update doc comment markup documentation.  Do that now.

Co-developed-by: John Snow <jsnow@redhat.com>
Signed-off-by: John Snow <jsnow@redhat.com>
Signed-off-by: Markus Armbruster <armbru@redhat.com>
Message-ID: <20250731054044.4011789-4-armbru@redhat.com>

docs/devel/qapi-code-gen.rst

@@ -943,9 +943,14 @@ The usual ****strong****, *\*emphasized\** and ````literal```` markup
 should be used.  If you need a single literal ``*``, you will need to
 backslash-escape it.
 
-Use ``@foo`` to reference a name in the schema.  This is an rST
-extension.  It is rendered the same way as ````foo````, but carries
-additional meaning.
+Use ```foo``` to reference a definition in the schema.  This generates
+a link to the definition.  In the event that such a cross-reference is
+ambiguous, you can use `QAPI cross-reference roles
+<QAPI-domain-cross-references>` to disambiguate.
+
+Use @foo to reference a member description within the current
+definition.  This is an rST extension.  It is currently rendered the
+same way as ````foo````, but carries additional meaning.
 
 Example::
 

docs/devel/qapi-domain.rst

@@ -375,6 +375,7 @@ Will allow you to add arbitrary field lists in QAPI directives::
 
       :see also: Lorem ipsum, dolor sit amet ...
 
+.. _QAPI-domain-cross-references:
 
 Cross-references
 ================