[docs] Update rbac_utils.rst documentation

This PS updates the rbac_utils.rst documentation to replace
occurrences of switch_role (which has been deprecated) with
override_role. The documentation has also been updated to
include correct and incorrect examples for how to use
the override_role contextmanager.

Change-Id: Ia68a58267cda2c31af0e296aba32b2f949788fd7
1 file changed