ethtool: Make struct ethtool_rxnfc kernel-doc more self-consistent

Refer consistently to 'classification rules' or just 'rules' rather
than 'filter specifications' or 'filter rules'.

Refer consistently to rule 'locations' and not 'indices'.

Signed-off-by: Ben Hutchings <bhutchings@solarflare.com>
Signed-off-by: David S. Miller <davem@davemloft.net>