open: make non-POSIX errno value more apparent

In the errno list, add an explicit note and reference to the note in the
STANDARDS section.

When O_NOFOLLOW is specified and the target is a symbolic link FreeBSD
sets errno to a value different than that specified by POSIX. Commit
295159dfa3ed added a note to this effect, but I missed it when reading
through the list of errno values.

PR: 214633
Reviewed by: markj
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D43618