message: docs: Update x-envoy-upstream-service-time description to be more accurate
diff: @@ -360,9 +360,9 @@ HTTP response headers set on downstream responses
  x-envoy-upstream-service-time
  ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  -Contains the time in milliseconds spent by the upstream host processing the request. This is useful
  -if the client wants to determine service time compared to network latency. This header is set on
  -responses.
  +Contains the time in milliseconds spent by the upstream host processing the request and the network
  +latency between Envoy and upstream host. This is useful if the client wants to determine service time
  +compared to network latency between client and Envoy. This header is set on responses.
  .. _config_http_filters_router_x-envoy-overloaded_set:
  