cr_bsd_visible(9): New man page

Approved by: re (gjb)
Reviewed by: bcr, pauamma_gundo.com
Sponsored by: Kumacom SAS
Differential Revision: https://reviews.freebsd.org/D40632

(cherry picked from commit 0d6bf73c4f20e6ed719c29c1b382d24bb0a81a2f)
(cherry picked from commit 3fa9a2a9177c5b4d8af7afecf0d74f345534a82c)