ethtool: Fill out and update comment for struct ethtool_ops

Briefly document all operations (except get_rx_ntuple), including
whether they may return an error code and whether they are deprecated.
Also mention some things that should be handled by the ethtool core
rather than by drivers.

Briefly document general requirements for callers.

Signed-off-by: Ben Hutchings <bhutchings@solarflare.com>