qapi: Implement boxed event argument documentation

Generate a reference "Arguments: the members of ...", just like we do
for commands since commit c2dd311cb7 "qapi2texi: Implement boxed
argument documentation".

No change to generated QMP documentation; we don't yet use boxed
events outside tests/.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Message-Id: <20191024110237.30963-7-armbru@redhat.com>
4 files changed