Add docstrings for check classes
Add docstrings for check classes

The check classes don't have docstrings, which breaks references
to those classes from other docstrings.  This moves the existing
__call__() method docstrings to be class docstrings to allow the
reference links to work.

Change-Id: Ib8adc3296471f8dd3ee52554d1b0e3b8e751d20d

citrix-openstack-build/oslo.policy,openstack/oslo.policy