net: ethtool: Document get_rxfh_context and set_rxfh_context ethtool ops

ethtool ops get_rxfh_context and set_rxfh_context are used to create,
remove and access parameters associated to RSS contexts, in a similar
fashion to get_rxfh and set_rxfh.

Add a small descritopn of these callbacks in the struct ethtool_ops doc.

Signed-off-by: Maxime Chevallier <maxime.chevallier@bootlin.com>
Signed-off-by: David S. Miller <davem@davemloft.net>