Add documentation describing the drivers use of ethtool ntuple filters,
including the limitations that it has due to hardware, as well as how it
reads and parses the user-def data block.

Signed-off-by: Jacob Keller <jacob.e.keller@intel.com>
Tested-by: Andrew Bowers <andrewx.bowers@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>