refactor: update part of resource identifier

proto/api/v1/reaction_service.proto

@@ -11,6 +11,8 @@ message Reaction {
   // Format: users/{user}
   string creator = 2;
 
+  // The content identifier.
+  // For memo, it should be the `Memo.name`.
   string content_id = 3;
 
   string reaction_type = 4;