doc: improve man pages related to bos restricted mode

Mention the restrictmode entry and the commands for setting and
querying it in the BosConfig man page, and add/fix cross references
between the BosConfig, bos, bos_getrestricted and bos_setrestricted
ones.

Reviewed-on: http://gerrit.openafs.org/10874
Reviewed-by: Ken Dreyer <ktdreyer@ktdreyer.com>
Reviewed-by: Benjamin Kaduk <kaduk@mit.edu>
Reviewed-by: Jeffrey Altman <jaltman@your-file-system.com>
Tested-by: Jeffrey Altman <jaltman@your-file-system.com>
(cherry picked from commit eee0bf5871944d919951cc8b7b4908ee909c3b62)

Change-Id: I25d2f23d75a9074ae478f86296bb13c1b2dda95b
Reviewed-on: http://gerrit.openafs.org/10883
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Michael Meffie <mmeffie@sinenomine.net>
Reviewed-by: Andrew Deason <adeason@sinenomine.net>
Reviewed-by: Stephan Wiesand <stephan.wiesand@desy.de>