devref: document API status for neutron.openstack.common.*
Make sure we document the fact that neutron.openstack.common.* contents
are not meant to be used by external repositories (except, temporarily,
*aas repos).
If I could bootstrap the oslo-incubator subtree from scratch, I would
put it under neutron._openstack, to indicate that it's for internal
usage only. But we can't do it now, so instead I update devref.