message: docs: update upstream network filters description
diff: @@ -4,8 +4,10 @@ Upstream network filters
  ========================
  Upstream clusters provide an ability to inject network level (L3/L4)
  -:ref:`filters <arch_overview_network_filters>`. The filters apply to the
  -connection to the upstream hosts, using the same API presented by listeners for
  -the downstream connections. The write callbacks are invoked for any chunk of
  -data sent to the upstream host, and the read callbacks are invoked for data
  +filters. It should be noted that a network filter needs to
  +be registered in code as an upstream filter before usage. Currently,
  +there are no upstream filters available in Envoy out of the box.
  +The filters apply to the connection to the upstream hosts, using the same API presented by listeners for
  +the downstream connections. The write-callbacks are invoked for any chunk of
  +data sent to the upstream host, and the read-callbacks are invoked for data
  received from the upstream host.
  