qapi: Conjure up QAPIDoc.ArgSection for undocumented members

qapi: Conjure up QAPIDoc.ArgSection for undocumented members

qapi2texi.py already conjures up ArgSections for undocumented
enumeration values, in texi_enum.  Drop that, and conjure them up for
all kinds of "arguments" (enumeration values, object and alternate
type members) in qapi.py instead.

Take care to keep generated documentation exactly the same for now.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Message-Id: <1489582656-31133-16-git-send-email-armbru@redhat.com>