usb: add decriptor of persist fail for some morph usb devices

Some morph devices will morph after being reset and cause persist
to fail. This patch is to add descriptor about this in the
Document/usb/persist.txt
Signed-off-by: Lan Tianyu <tianyu.lan@intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Documentation/usb/persist.txt

@@ -155,6 +155,9 @@ If the kernel gets fooled in this way, it's almost certain to cause
 data corruption and to crash your system.  You'll have no one to blame
 but yourself.
 
+For those devices with avoid_reset_quirk attribute being set, persist
+maybe fail because they may morph after reset.
+
 YOU HAVE BEEN WARNED!  USE AT YOUR OWN RISK!
 
 That having been said, most of the time there shouldn't be any trouble