qapi: Belatedly document modular code generation

We generate code for built-ins and sub-modules into separate files
since commit cdb6610ae42 and 252dc3105fc (v2.12.0).  Both commits
neglected to update documentation.  Do that now.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Message-Id: <20190214152251.2073-2-armbru@redhat.com>
1 file changed