USB: documentation update for usb_unlink_urb

This patch (as936) updates the kerneldoc for usb_unlink_urb.  The
explanation of how endpoint queues are meant to work is now clearer
and in better agreement with reality.

Signed-off-by: Alan Stern <stern@rowland.harvard.edu>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>