Adder and remover sidenote

| Q             | A
| ------------- | ---
| Doc fix?      | yes
| New docs?     | no
| Applies to    | all
| Fixed tickets | none

There are cases when you only want either adder or remover, important to know both of them need to be found if any is to work.

Author: kshishkin

cookbook/form/form_collections.rst

@@ -462,7 +462,8 @@ these new ``Tag`` objects easier (especially if you're using Doctrine, which
 we talk about next!).
 
 .. caution::
-
+    Create **both** methods, otherwise none of them will be used.
+    
     If no ``addTag`` **and** ``removeTag`` method is found, the form will
     still use ``setTag`` even if ``by_reference`` is ``false``. You'll learn
     more about the ``removeTag`` method later in this article.