- Remove the warning about NULL filesystem. It is stable and safe to use in   both 6.x and 7.x. This is based on feedbacks on this thread
- Remove the warning about NULL filesystem. It is stable and safe to use in
  both 6.x and 7.x. This is based on feedbacks on this thread

  http://docs.freebsd.org/cgi/getmsg.cgi?fetch=81818+0+current/freebsd-stable

  and my use it on 6.x.

MFC after:   	3 days

- Update the warning about UNION filesystem. It is now actively maintained,
  although there are still some issues being resolved.

Reviewed by:	freebsd-stable@, kris, bmah
Approved by:	re (bmah)

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