docs: ethtool-netlink: document interface for MAC Merge layer

Show details about the structures passed back and forth related to MAC
Merge layer configuration, state and statistics. The rendered htmldocs
will be much more verbose due to the kerneldoc references.

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