DAFS: document the limits of -vhashsize

DAFS: document the limits of -vhashsize

You can only specify a -vhashsize between 6 and 28 (inclusive).
Document that in the dafileserver man page.

Reviewed-on: http://gerrit.openafs.org/2650
Reviewed-by: Derrick Brashear <shadow@dementia.org>
Tested-by: Derrick Brashear <shadow@dementia.org>
(cherry picked from commit 7e12632cd2c8e07baa5e6c8aecd5311870771133)
Change-Id: Ibe7da6f9916c1664050c28d136fe887b35a540ba
Reviewed-on: http://gerrit.openafs.org/2936