net: update the usage of CHECKSUM_UNNECESSARY

As suggested by Ben, this adds the clarification on the usage of
CHECKSUM_UNNECESSARY on the outgoing patch. Also add the usage
description of NETIF_F_FCOE_CRC and CHECKSUM_UNNECESSARY
for the kernel FCoE protocol driver.

This is a follow-up to the following:
http://patchwork.ozlabs.org/patch/147315/Signed-off-by: Yi Zou <yi.zou@intel.com>
Cc: Ben Hutchings <bhutchings@solarflare.com>
Cc: Jeff Kirsher <jeffrey.t.kirsher@intel.com>
Cc: www.Open-FCoE.org <devel@open-fcoe.org>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/linux/skbuff.h

@@ -94,6 +94,13 @@
  *			  about CHECKSUM_UNNECESSARY. 8)
  *	NETIF_F_IPV6_CSUM about as dumb as the last one but does IPv6 instead.
  *
+ *	UNNECESSARY: device will do per protocol specific csum. Protocol drivers
+ *	that do not want net to perform the checksum calculation should use
+ *	this flag in their outgoing skbs.
+ *	NETIF_F_FCOE_CRC  this indicates the device can do FCoE FC CRC
+ *			  offload. Correspondingly, the FCoE protocol driver
+ *			  stack should use CHECKSUM_UNNECESSARY.
+ *
  *	Any questions? No questions, good. 		--ANK
  */