Document cert forwarding in the XFCC header. (#510)

Signed-off-by: William Thurston <wthurston@netflix.com>

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

@@ -212,8 +212,9 @@ message HttpConnectionManager {
     // Whether to forward the SAN of the client cert. Defaults to false.
     google.protobuf.BoolValue san = 2;
 
-    // [#not-implemented-hide:]
+    // Whether to forward the entire client cert in URL encoded PEM format. This will appear in the
-    // Whether to forward the entire client cert in base64 encoded format. Defaults to false.
+    // XFCC header comma separated from other values with the value Cert="PEM".
+    // Defaults to false.
     bool cert = 3;
   };