minor #13439 Shortening Doctrine Note (ThomasLandauer)

This PR was squashed before being merged into the 3.4 branch (closes #13439).

Discussion
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Shortening Doctrine Note

Follow up of #13437

Commits
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0f387d9 Shortening Doctrine Note

Author: javiereguiluz

form/form_collections.rst

@@ -11,13 +11,7 @@ that Task, right inside the same form.
 
 .. note::
 
-    In this article, it's loosely assumed that you're using Doctrine as your
-    database store. But if you're not using Doctrine, it's all very similar.
-    There are only a few parts of this tutorial that really care about "persistence".
-
-    If you *are* using Doctrine, you'll need to add the Doctrine metadata,
-    including the ``ManyToMany`` association mapping definition on the Task's
-    ``tags`` property.
+    The example presented here is based on Doctrine.
 
 Let's start by creating a ``Task`` entity::