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.

Reviewed by: bcr, pauamma_gundo.com
MFC after: 2 weeks
Sponsored by: Kumacom SAS
Differential Revision: https://reviews.freebsd.org/D40638