realpath: Belatedly document POSIX conformance

We've been mostly POSIX-conforming since r236400 and fully since r240410,
which fixed a corner case where a missing non-leaf directory would be
reported as ENOTDIR instead of ENOENT.

Sponsored by: Klara, Inc.
Fixes: 7877ed7ce33e ("Avoid mapping ENOENT to ENOTDIR for non-existent path components.")
Reviewed by: ziaee, markj
Differential Revision: https://reviews.freebsd.org/D53027

(cherry picked from commit 7c66667d45e95af59f59e41ef169119a974a6be1)