doc: clarify setcrypt defaults

Change-Id: I0145ecc9935786204d215a11459d170f6220657b
Reviewed-on: http://gerrit.openafs.org/10111
Reviewed-by: Jason Edgecombe <jason@rampaginggeek.com>
Reviewed-by: Michael Laß <lass@mail.uni-paderborn.de>
Reviewed-by: Jeffrey Altman <jaltman@your-file-system.com>
Reviewed-by: Derrick Brashear <shadow@your-file-system.com>
Tested-by: Derrick Brashear <shadow@your-file-system.com>