update upstream filters description

Signed-off-by: Tsimafei Bredau <timofei.bredov@gmail.com>

docs/root/intro/arch_overview/upstream/upstream_filters.rst

@@ -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
+filters. It should be noted that network fitler 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.