ethtool: Update documentation of struct ethtool_pauseparam

Convert the inline comments to kernel-doc format.

Explicitly specify that non-zero autoneg is an error if link
autonegotiation is disabled.

Specify that pause capabilities should be advertised dependent on link
autonegotiation, not the autoneg flag here.  There is no way to
opt-out of pause frame autonegotiation, and this improves behaviour
when the link partner is configured to follow pause frame
autonegotiation and our interface is not.

Signed-off-by: Ben Hutchings <ben@decadent.org.uk>
Signed-off-by: David S. Miller <davem@davemloft.net>