fix HeaderAppendAction comment (#31489)

Signed-off-by: wangkai19 <wangkai19@corp.netease.com>

Mirrored from https://github.com/envoyproxy/envoy @ f313fc6db74564ed1fb37451f9270e3a2975111d

envoy/config/core/v3/base.proto

@@ -346,9 +346,12 @@ message HeaderValueOption {
 
   // Describes the supported actions types for header append action.
   enum HeaderAppendAction {
-    // This action will append the specified value to the existing values if the header
+    // If the header already exists, this action will result in:
-    // already exists. If the header doesn't exist then this will add the header with
+    //
-    // specified key and value.
+    // - Comma-concatenated for predefined inline headers.
+    // - Duplicate header added in the ``HeaderMap`` for other headers.
+    //
+    // If the header doesn't exist then this will add new header with specified key and value.
     APPEND_IF_EXISTS_OR_ADD = 0;
 
     // This action will add the header if it doesn't already exist. If the header