qapi: Reject section markup in definition documentation

Section markup in definition documentation makes no sense and can
produce invalid Texinfo.  Reject.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Message-Id: <20200320091805.5585-2-armbru@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>

scripts/qapi/parser.py

@@ -282,6 +282,11 @@ class QAPISchemaParser:
                 doc.end_comment()
                 self.accept()
                 return doc
+            if self.val.startswith('# ='):
+                if doc.symbol:
+                    raise QAPIParseError(
+                        self,
+                        "unexpected '=' markup in definition documentation")
             doc.append(self.val)
             self.accept(False)