doc: Correct volume size CAUTIONS notes

doc: Correct volume size CAUTIONS notes

2 terabytes is 2^41 bytes, not 2^31 bytes. Also clarify that volumes
can be much larger than this.

FIXES 130188

Reviewed-on: http://gerrit.openafs.org/7598
Reviewed-by: Jeffrey Altman <jaltman@your-file-system.com>
Tested-by: Jeffrey Altman <jaltman@your-file-system.com>
(cherry picked from commit 300cbb28b08c2fd3709942cc6c48e7d9369a3394)

Change-Id: I762e18e83d4557c8fd4db3601426bcaaa845199a
Reviewed-on: http://gerrit.openafs.org/7674
Reviewed-by: Jeffrey Altman <jaltman@your-file-system.com>
Tested-by: Jeffrey Altman <jaltman@your-file-system.com>