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.

Change-Id: I938ef4c43c1a248335f09975c454b36f7570782c
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>