docs/sphinx: remove special parsing for freeform sections

Remove the QAPI doc section heading syntax, use plain rST section
headings instead.

Tests and documentation are updated to match.

Interestingly, Plain rST headings work fine before this patch, except
for over- and underlining with '=', which the doc parser rejected as
invalid QAPI doc section heading in free-form comments.

Signed-off-by: John Snow <jsnow@redhat.com>
Message-ID: <20250618165353.1980365-5-jsnow@redhat.com>
Reviewed-by: Markus Armbruster <armbru@redhat.com>
[Add more detail to commit message]
Signed-off-by: Markus Armbruster <armbru@redhat.com>

scripts/qapi/parser.py

@@ -597,22 +597,15 @@ class QAPISchemaParser:
             # Free-form documentation
             doc = QAPIDoc(info)
             doc.ensure_untagged_section(self.info)
-            first = True
             while line is not None:
                 if match := self._match_at_name_colon(line):
                     raise QAPIParseError(
                         self,
                         "'@%s:' not allowed in free-form documentation"
                         % match.group(1))
-                if line.startswith('='):
-                    if not first:
-                        raise QAPIParseError(
-                            self,
-                            "'=' heading must come first in a comment block")
                 doc.append_line(line)
                 self.accept(False)
                 line = self.get_doc_line()
-                first = False
 
         self.accept()
         doc.end()