Update rbac_rule_validation docstrings

This commit updates, clarifies and elaborates on pre-existing
docstrings in ``rbac_rule_validation``. Currently, some of the
documentation is either unclear, slightly out of date, or not
informative enough. This commit seeks to change those documentation
deficiencies.

Change-Id: I06092f6c7693833ea2e1e48513b5498a0e044642
1 file changed