Correctly document the AFS client setuid defaults

Correctly document the AFS client setuid defaults

AFS no longer honors setuid status by default.  Update the admin
guide documentation appropriately and add a warning recommending
against enabling setuid status given the limitations of the current
AFS protocol.

Reformat this section of the admin guide to make it easier to
maintain.

Change-Id: I6ea5859037d7d194df801f1a76583257cfc8bbe8
Reviewed-on: http://gerrit.openafs.org/4311
Reviewed-by: Jeffrey Altman <jaltman@openafs.org>
Reviewed-by: Derrick Brashear <shadow@dementia.org>
Tested-by: Derrick Brashear <shadow@dementia.org>