docs: net: include the new ethtool pause stats in the stats doc

Tell people that there now is an interface for querying pause frames.
A little bit of restructuring is needed given this is a first source
of such statistics.

Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Reviewed-by: Saeed Mahameed <saeedm@nvidia.com>
Signed-off-by: David S. Miller <davem@davemloft.net>