feature #8495 [Form] Document disabling the usage of globally defined form themes (emodric, javiereguiluz)

This PR was merged into the 3.4 branch.

Discussion
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[Form] Document disabling the usage of globally defined form themes

This PR documents the option to disable the usage of globally defined form themes. This is suggested as a feature in symfony/symfony#22610 (reviewed, but not yet merged)

Commits
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14ef7ae Minor reword
8d45f83 Document disabling the usage of globally defined form themes

Author: weaverryan

form/form_customization.rst

@@ -343,6 +343,38 @@ name of all the templates as an array using the ``with`` keyword:
 The templates can also be located in different bundles, use the Twig namespaced
 path to reference these templates, e.g. ``@AcmeFormExtra/form/fields.html.twig``.
 
+Disabling usage of globally defined themes
+..........................................
+
+Sometimes it is useful to disable usage of globally defined form themes in order
+to have more control over rendering of a form. You might want this, for example,
+when creating admin interface for a bundle which can be installed on a wide range
+of Symfony apps and you can't control what themes are defined globally.
+
+You can do this by including the ``only`` keyword after the list of used form
+themes:
+
+.. code-block:: html+twig
+
+    {% form_theme form with ['common.html.twig', 'form/fields.html.twig'] only %}
+
+    {# ... #}
+
+.. caution::
+
+    When using the ``only`` keyword, none of Symfony's built-in form themes
+    (``form_div_layout.html.twig``, etc.) will be applied. In order to render
+    your forms correctly, you need to either provide a full-featured form theme
+    yourself, or extend one of the built-in form themes with Twig's ``use``
+    keyword instead of ``extends`` to reuse the original theme contents.
+
+    .. code-block:: html+twig
+
+        {# app/Resources/views/common.html.twig #}
+        {% use "form_div_layout.html.twig" %}
+
+        {# ... #}
+
 Child Forms
 ...........