ethtool: Expand documentation of struct ethtool_drvinfo

Replace the inline comments (and some others below) with a full
explanation of the semantics, in kernel-doc format.  Specify which
strings may be empty.  Document the relationship with other commands.

Replace the 'deprecation' of some fields with a proper explanation of
the conversion to generalised string sets, as userland programs may
not be able to assume that ETHTOOL_GSSET_INFO is available.

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