ethtool: Expand documentation of struct ethtool_cmd

struct ethtool_cmd has very limited documentation; it contains
several obscure or obsolete fields and several with non-obvious
interpretation.

Replace the inline comments (and some others below) with a full
explanation of the semantics as well as I understand them, in
kernel-doc format.  Formally deprecate some fields that seem to be of
historical use only.

Extend the comment about 32/64-bit compatibility to cover all
ethtool structures.

Signed-off-by: Ben Hutchings <ben@decadent.org.uk>
Signed-off-by: David S. Miller <davem@davemloft.net>