minor #4253 Adder and remover sidenote (kshishkin)

This PR was merged into the 2.3 branch.

Discussion
----------

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.

Commits
-------

59b932d Merging emphasized notes
bdd8325 Adder and remover sidenote

cookbook/form/form_collections.rst

@@ -463,9 +463,9 @@ we talk about next!).
 
 .. caution::
 
-    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.
+    You have to create **both** ``addTag`` and ``removeTag`` methods, 
+    otherwise the form will still use ``setTag`` even if ``by_reference`` is ``false``. 
+    You'll learn more about the ``removeTag`` method later in this article.
 
 .. sidebar:: Doctrine: Cascading Relations and saving the "Inverse" side