docs/devel/qapi-code-gen: Document 'features' introspection

Commit 6a8c0b5102 "qapi: Add feature flags to struct types" neglected
to update section "Client JSON Protocol introspection", and commit
23394b4c39 "qapi: Add feature flags to commands" didn't either.  Make
up for that.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Message-Id: <20200317115459.31821-5-armbru@redhat.com>
1 file changed