Add documentation for the i40e_get_stats_count, i40e_get_stat_strings
and i40e_get_ethtool_stats explaining that the number and ordering of
statistics must remain constant for a given netdevice.
Signed-off-by: NJacob Keller <jacob.e.keller@intel.com>
Tested-by: NAndrew Bowers <andrewx.bowers@intel.com>
Signed-off-by: NJeff Kirsher <jeffrey.t.kirsher@intel.com>