followup: Include spec/discussion references

This patchset follows up on [0] by addressing
feedback left in the review section related to
adding background references:

* https://review.openstack.org/#/c/382672/
* https://specs.openstack.org/openstack/qa-specs/specs/tempest/rbac-policy-testing.html

For the purposes of corrborating the documentation
content related to "Why are these tests not in Tempest?"

[0] https://review.openstack.org/#/c/580903/4

Change-Id: I69ca13cf16f0d033419a7ea086bf0605682c2dbc
1 file changed