_Exit(3): document implementation

Remove a useless note about unlinking temporary files, they are unlinked
in tmpfile(3) [1]. Add a note about __cxa_atexit().

Explain exactly what are the FreeBSD implementation differences between
exit() and _Exit().

Noted by: markj [1]
Reviewed by: emaste, markj
Sponsored by: The FreeBSD Foundation
MFC after: 3 days
Differential revision: https://reviews.freebsd.org/D31425