qapi: add 'If:' section to generated documentation

The documentation is generated only once, and doesn't know C
pre-conditions. Add 'If:' sections for top-level entities.

Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Reviewed-by: Markus Armbruster <armbru@redhat.com>
Message-Id: <20180703155648.11933-13-marcandre.lureau@redhat.com>
Signed-off-by: Markus Armbruster <armbru@redhat.com>