Provide a way to indicate move/removal of a module symbol

[Brant Knudson]

In this case we want to indicate that this module symbol is going to be removed: https://review.openstack.org/#/c/205687/2/keystoneclient/client.py

Should be an example in the docs and a unit test.

[ChangBo Guo(gcb) (glongwave)]

I think we have covered this in https://github.com/openstack/debtcollector/blob/master/doc/source/user/usage.rst