cr_canseeotheruids(9): Revamp, mark as internal

Significantly clarify. Replace references to cr_canseeothergids(9) by
ones to cr_bsd_visible(9).

Reviewed by: bcr, mhorne
MFC after: 2 weeks
Sponsored by: Kumacom SAS
Differential Revision: https://reviews.freebsd.org/D40635

(cherry picked from commit 4ddd253b38dff8725555355cc1b5238b1bbfd380)