ethdev: clarify extended statistics documentation
ethdev: clarify extended statistics documentation

Reword the API documentation of ethdev xstats.

Signed-off-by: Olivier Matz <dfcc1510895413197abb4336e39bb0a3906cba71@6wind.com>
Acked-by: Remy Horton <34c343e92f0e27f7a8e26a1d8d2146ae7c7dd1a3@intel.com>

john-mcnamara-intel/dpdk,john-mcnamara-intel/dpdk,john-mcnamara-intel/dpdk,john-mcnamara-intel/dpdk