qapi: Merge adjacent untagged sections

The parser mostly doesn't create adjacent untagged sections, and
merging the ones it does create is hardly worth the bother.  I'm doing
it to avoid behavioral change in the next commit.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Message-ID: <20240216145841.2099240-14-armbru@redhat.com>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>

scripts/qapi/parser.py

@@ -719,6 +719,9 @@ class QAPIDoc:
         self._start_symbol_section(self.features, name)
 
     def _start_section(self, tag: Optional[str] = None) -> None:
+        if not tag and not self._section.tag:
+            # extend current section
+            return
         if tag in ('Returns', 'Since') and self.has_section(tag):
             raise QAPIParseError(self._parser,
                                  "duplicated '%s' section" % tag)