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>