doc: bos manpage fixes

Add missing documentation for the -mode option.

Reviewed-on: http://gerrit.openafs.org/10390
Reviewed-by: Ken Dreyer <ktdreyer@ktdreyer.com>
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@your-file-system.com>
(cherry picked from commit 070230ab76e1df338db3f2a7971111ca976a0c1a)

Change-Id: I0780fc350840631d55d60c21e6223020ca9a845f
Reviewed-on: http://gerrit.openafs.org/10954
Reviewed-by: Andrew Deason <adeason@sinenomine.net>
Reviewed-by: Chas Williams - CONTRACTOR <chas@cmf.nrl.navy.mil>
Reviewed-by: Stephan Wiesand <stephan.wiesand@desy.de>
Tested-by: Stephan Wiesand <stephan.wiesand@desy.de>