minor #6059 Add best practice about the Form type namespace (WouterJ)

This PR was merged into the 2.3 branch.

Discussion
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Add best practice about the Form type namespace

| Q | A
| --- | ---
| Doc fix? | no
| New docs? | yes
| Applies to | all
| Fixed tickets | -

/cc @javiereguiluz @weaverryan as you are the main guys behind this guide.

Commits
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0275bef Add best practice about the Form type namespace

best_practices/forms.rst

@@ -49,11 +49,17 @@ form in its own PHP class::
         }
     }
 
+.. best-practice::
+
+    Put the form type classes in the ``AppBundle\Form`` namespace, unless you
+    use other custom form classes like data transformers.
+
 To use the class, use ``createForm`` and instantiate the new class::
 
-    use AppBundle\Form\PostType;
     // ...
+    use AppBundle\Form\PostType;
 
+    // ...
     public function newAction(Request $request)
     {
         $post = new Post();