Make it clear that MFS can be used for the noperm permission holding DESTDIR.

Make it clear that MFS can be used for the noperm permission holding DESTDIR.
(If anyone needs a bit more handholding than is in release(8), the faq page
https://www.openbsd.org/faq/faq5.html#Release has more information about
setting this up).

Much simplified from, but inspired by, various diffs from ians@.  ok tb@ jmc@