api.rst: note about duplicate add_meta calls

If you call add_meta twice with the same meta class you get
an IntegrityError:

    IntegrityError: duplicate key value violates unique constraint "myapp_mymetaclass_revision_id_key"
    DETAIL:  Key (revision_id)=(14203) already exists.

Make a note in the docs that you shouldn't call add_meta
twice during a revision context with the same meta class.