Update permissions description to reflect separation of purge progress bar and manual purge block

Permission Purge on-screen still reads
Process purges visually using a progress bar and grant access to the editorial manual purge blocks.

As of 7.x-1.2, this would now correctly read:
Process purges visually using a progress bar.

Or better:
Allow user to see purges as they are processed. Includes text and a progress bar.