Documentation: add a guideline for hard_start_xmit method

Add a guideline not to modify SKBs.
Signed-off-by: Matti Linnanvuori <mattilinnanvuori@yahoo.com>
Signed-off-by: Jeff Garzik <jeff@garzik.org>

Documentation/networking/driver.txt

@@ -61,7 +61,10 @@ Transmit path guidelines:
 2) Do not forget to update netdev->trans_start to jiffies after
    each new tx packet is given to the hardware.
 
-3) Do not forget that once you return 0 from your hard_start_xmit
+3) A hard_start_xmit method must not modify the shared parts of a
+   cloned SKB.
+
+4) Do not forget that once you return 0 from your hard_start_xmit
    method, it is your driver's responsibility to free up the SKB
    and in some finite amount of time.