i40e: document drivers use of ntuple filters

i40e: document drivers use of ntuple filters

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>
Orabug: 26132523
(cherry picked from commit 55877012d5588ce7427919d6b869922f1a5f60bc)
Signed-off-by: Jack Vogel <jack.vogel@oracle.com>
Reviewed-by: Brian Maly <brian.maly@oracle.com>