docs: clarifying upstream filter docs (#16803)

Risk Level: n/a
Testing: n/a
Docs Changes: yes
Addressing concerns from #16599

Signed-off-by: Alyssa Wilk <alyssar@chromium.org>

Mirrored from https://github.com/envoyproxy/envoy @ e32c9e3c6a3ba546d70d31c27996f5b18c431410

envoy/config/cluster/v3/filter.proto

@@ -20,7 +20,8 @@ message Filter {
   option (udpa.annotations.versioning).previous_message_type = "envoy.api.v2.cluster.Filter";
 
   // The name of the filter to instantiate. The name must match a
-  // :ref:`supported filter <config_network_filters>`.
+  // supported upstream filter. Note that Envoy's :ref:`downstream network
+  // filters <config_network_filters>` are not valid upstream filters.
   string name = 1 [(validate.rules).string = {min_len: 1}];
 
   // Filter specific configuration which depends on the filter being

envoy/config/cluster/v4alpha/filter.proto

@@ -20,7 +20,8 @@ message Filter {
   option (udpa.annotations.versioning).previous_message_type = "envoy.config.cluster.v3.Filter";
 
   // The name of the filter to instantiate. The name must match a
-  // :ref:`supported filter <config_network_filters>`.
+  // supported upstream filter. Note that Envoy's :ref:`downstream network
+  // filters <config_network_filters>` are not valid upstream filters.
   string name = 1 [(validate.rules).string = {min_len: 1}];
 
   // Filter specific configuration which depends on the filter being