minor symfony#12281 Added a caution message about unit testing form types (javiereguiluz)

This PR was squashed before being merged into the 3.4 branch (closes symfony#12281).

Discussion
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Added a caution message about unit testing form types

Fixes symfony#8972.

Commits
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30e2e88 Added a caution message about unit testing form types

Author: javiereguiluz

form/unit_testing.rst

@@ -4,6 +4,13 @@
 How to Unit Test your Forms
 ===========================
 
+.. caution::
+
+    This article is intended for developers who create
+    :doc:`custom form types </form/create_custom_field_type>`. If you are using
+    the :doc:`built-in Symfony form types </reference/forms/types>` or the form
+    types provided by third-party bundles, you don't need to unit test them.
+
 The Form component consists of 3 core objects: a form type (implementing
 :class:`Symfony\\Component\\Form\\FormTypeInterface`), the
 :class:`Symfony\\Component\\Form\\Form` and the