qapi: Improve error message for empty doc sections

Improve the message for an empty tagged section from

    empty doc section 'Note'

to

    text required after 'Note:'

and the message for an empty argument or feature description from

    empty doc section 'foo'

to

    text required after '@foo:'

Improve the error position to refer to the beginning of the empty
section instead of its end.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Message-ID: <20240216145841.2099240-8-armbru@redhat.com>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>

docs/sphinx/qapidoc.py

@@ -239,8 +239,8 @@ class QAPISchemaGenRSTVisitor(QAPISchemaVisitor):
         seen_item = False
         dlnode = nodes.definition_list()
         for section in doc.features.values():
-            dlnode += self._make_dlitem([nodes.literal('', section.name)],
-                                        section.text)
+            dlnode += self._make_dlitem(
+                [nodes.literal('', section.member.name)], section.text)
             seen_item = True
 
         if not seen_item: