[FrameworkBundle] add support for prioritizing form type extension tags

Author: dmaicher

form/create_form_type_extension.rst

@@ -128,6 +128,14 @@ The ``extended_type`` key of the tag is the type of field that this extension sh
 be applied to. In your case, as you want to extend the ``Symfony\Component\Form\Extension\Core\Type\FileType``
 field type, you will use that as the ``extended_type``.
 
+.. tip::
+
+    There is an optional tag attribute called ``priority``, which 
+    defaults to ``0`` and controls the order in which the form  
+    type extensions are loaded (the higher the priority, the earlier 
+    an extension is loaded). This is useful when you need to guarantee 
+    that one extension is loaded before or after another extension.
+
 Adding the extension Business Logic
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