docs: ethtool: document ETHTOOL_A_STATS_SRC and ETHTOOL_A_PAUSE_STATS_SRC

Two new netlink attributes were added to PAUSE_GET and STATS_GET and
their replies. Document them.

Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Signed-off-by: David S. Miller <davem@davemloft.net>