First draft of documentation for AT/O_BENEATH handling of the absolute
paths.
It was decided that committing the code and drafting of the man page
update is better than allowing the code to rot until wordsmithing
happens.
Reviewed by: jilles (previous version)
Discussed with: brooks, jilles, emaste
Sponsored by: The FreeBSD Foundation
MFC after: 1 week
Differential revision: https://reviews.freebsd.org/D17714