USB documentation: explain lifetime rules for unlinking URBs

This patch (as1534c) updates the documentation for usb_unlink_urb and
related functions.  It explains that the caller must prevent the URB
being unlinked from getting deallocated while the unlink is taking
place.

Signed-off-by: Alan Stern <stern@rowland.harvard.edu>
CC: Ming Lei <tom.leiming@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>