ethtool: clarify the ethtool FEC interface

The definition of the FEC driver interface is quite unclear.
Improve the documentation.

This is based on current driver and user space code, as well
as the discussions about the interface:

RFC v1 (24 Oct 2016): https://lore.kernel.org/netdev/1477363849-36517-1-git-send-email-vidya@cumulusnetworks.com/
 - this version has the autoneg field
 - no active_fec field
 - none vs off confusion is already present

RFC v2 (10 Feb 2017): https://lore.kernel.org/netdev/1486727004-11316-1-git-send-email-vidya@cumulusnetworks.com/
 - autoneg removed
 - active_fec added

v1 (10 Feb 2017): https://lore.kernel.org/netdev/1486751311-42019-1-git-send-email-vidya@cumulusnetworks.com/
 - no changes in the code

v1 (24 Jun 2017):  https://lore.kernel.org/netdev/1498331985-8525-1-git-send-email-roopa@cumulusnetworks.com/
 - include in tree user

v2 (27 Jul 2017): https://lore.kernel.org/netdev/1501199248-24695-1-git-send-email-roopa@cumulusnetworks.com/



Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>