docs/api: hide represent_ipv4_remote_address_as_ipv4_mapped_ipv6. (#6432)

This is not implemented yet.

Fixes #6405.

Signed-off-by: Harvey Tuch <htuch@google.com>

Mirrored from https://github.com/envoyproxy/envoy @ 4ed3afebe9801e40095a68fa135ccf10a474818a

envoy/config/filter/network/http_connection_manager/v2/http_connection_manager.proto

@@ -345,6 +345,7 @@ message HttpConnectionManager {
   // :ref:`http_connection_manager.represent_ipv4_remote_address_as_ipv4_mapped_ipv6
   // <config_http_conn_man_runtime_represent_ipv4_remote_address_as_ipv4_mapped_ipv6>` for runtime
   // control.
+  // [#not-implemented-hide:]
   bool represent_ipv4_remote_address_as_ipv4_mapped_ipv6 = 20;
 
   // The configuration for HTTP upgrades.