doc: bos manpage fixes

Add missing documentation for the -mode option.

Change-Id: I360dc3d4cc6d7405feb655bbf2a05aee27a41d46
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>