ethtool: Complete kernel-doc comments for RX flow filter and hash control

There are now several interfaces within the ethtool API for getting
and setting RX flow filtering and hashing behaviour, most of which are
poorly documented.  This adds kernel-doc comments for all these
interfaces, based on the existing incomplete comments and on the
initial implementations.

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