Improve the documentation around that.
* The "-p pax", "-m mknod", and "-s" (mtree specfile) options should
always have been mutually exclusive. Now enforce that.
* The "-f" option didn't ever work with pax or mtree mode; document that,
and make it an error to try something that won't work.
* Make pax mode the default, if no other options prevent this, and if
"pax -w -M" appears to be usable.
* Add missing options to a usage message.
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