docs/devel/writing-qmp-commands: Update for modular QAPI

docs/devel/writing-qmp-commands: Update for modular QAPI

With modular code generation, putting stuff right into
qapi-schema.json is a bad idea.  Update writing-qmp-commands.txt
accordingly.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Message-Id: <20180211093607.27351-26-armbru@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Reviewed-by: Michael Roth <mdroth@linux.vnet.ibm.com>
Signed-off-by: Eric Blake <eblake@redhat.com>