generic proxy: add doc about the generic proxy (#28397)

Signed-off-by: wbpcode <wangbaiping@corp.netease.com>

Mirrored from https://github.com/envoyproxy/envoy @ e0bca625f98a0270a78e7c1aa787ca4241eb7d60

contrib/envoy/extensions/filters/network/generic_proxy/matcher/v3/matcher.proto

@@ -47,7 +47,6 @@ message PropertyMatchInput {
   string property_name = 1 [(validate.rules).string = {min_len: 1}];
 }
 
-// [#not-implemented-hide:]
 // Used to match an whole generic downstream request.
 message RequestMatchInput {
 }