Mention mirror_percent field in mirroring task (#4975)

* Mention mirror_percent field in mirroring task

* Apply suggestions from code review

Co-Authored-By: Adam Miller <1402860+adammil2000@users.noreply.github.com>

content/en/docs/tasks/traffic-management/mirroring/index.md

@@ -220,6 +220,7 @@ log entries for `v1` and none for `v2`:
         mirror:
           host: httpbin
           subset: v2
+        mirror_percent: 100
     EOF
     {{< /text >}}
 
@@ -231,6 +232,9 @@ log entries for `v1` and none for `v2`:
     Also, it is important to note that these requests are mirrored as "fire and
     forget", which means that the responses are discarded.
 
+    You can use the `mirror_percent` field to mirror a fraction of the traffic,
+    instead of mirroring all requests. If this field is absent, for compatibility with
+    older versions, all traffic will be mirrored.
 1. Send in traffic:
 
     {{< text bash >}}