doc: add --latencystats option in testpmd guide

[ upstream commit 8cd8f0a90bba2862d8044529e11276d6f6321192 ]

Option was added but never added to documentation.

Fixes: 62d3216 ("app/testpmd: add latency statistics calculation")

Signed-off-by: Stephen Hemminger <stephen@networkplumber.org>
Acked-by: Ferruh Yigit <ferruh.yigit@amd.com>

doc/guides/testpmd_app_ug/run_app.rst

@@ -418,6 +418,10 @@ The command line options are:
 
     Set the logical core N to perform bitrate calculation.
 
+*   ``--latencystats=N``
+
+    Set the logical core N to perform latency and jitter calculations.
+
 *   ``--print-event <unknown|intr_lsc|queue_state|intr_reset|vf_mbox|macsec|intr_rmv|dev_probed|dev_released|flow_aged|err_recovering|recovery_success|recovery_failed|all>``
 
     Enable printing the occurrence of the designated event. Using all will