doc: add statistics read frequency to ixgbe guide

doc: add statistics read frequency to ixgbe guide

This patch adds a note to the ixgbe PMD guide, stating
the minimum time that statistics must be polled from
the hardware in order to avoid register values becoming
saturated and "sticking" to the max value.

Reported-by: Jerry Zhang <jerry.zhang@intel.com>
Tested-by: Marcin Kerlin <marcinx.kerlin@intel.com>
Signed-off-by: Harry van Haaren <harry.van.haaren@intel.com>
Acked-by: Marcin Kerlin <marcinx.kerlin@intel.com>