Document use of GenericCacheLocation.

This was introduced by 1e5476a2f50cacbd349c43868701eeac6f056014.

Change-Id: Ie216dd56978d467483d4b80798cfd5d17ca49e48
Reviewed-by: Aaron McCarthy <mccarthy.aaron@gmail.com>
Reviewed-by: Alex Blasche <alexander.blasche@digia.com>