Document the managed permission updater operation
The method was explained on the [Design] page, but as the updater
is extended the design page would become obsolete.
Document the operation in the docstring of the plugin itself.
Design: http://www.freeipa.org/page/V3/Managed_Read_permissions#Default_Permission_Updater
Reviewed-By: Martin Kosek <mkosek@redhat.com>