[PATCH] Update udf documentation to reflect current state of read/write support

Change Documentation/filesystems/udf.txt from saying that read/write mounts
on cd media are not supported to instead state the current level of
support.  Specifically that it works fine on dvd+rw media and can be made
to work on cd-rw media via the pktcdvd device.

Cc: Peter Osterlund <petero2@telia.com>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>