p_candebug(9): cr_bsd_visible() impacts, misc fixes

Mention cr_bsd_visible(9). Remove references to cr_canseeothergids(9)
and cr_canseeotheruids(9), as well as indirect references not
immediately useful.

Fix description of credentials checks to match reality.

Re-order errors to match code's check order.

Approved by: re (gjb)
Reviewed by: bcr, pauamma_gundo.com
Sponsored by: Kumacom SAS
Differential Revision: https://reviews.freebsd.org/D40638

(cherry picked from commit eb94f24fab4b44f13ca045370d9fcf12ca8835f2)
(cherry picked from commit c29ee9e238b36ef8a7c4508582d4512cda885192)