feature symfony#4084 [Book][Forms] describe the allow_extra_fields form option (xabbuh)

This PR was submitted for the master branch but it was merged into the 2.6 branch instead (closes symfony#4084).

Discussion
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[Book][Forms] describe the allow_extra_fields form option

| Q             | A
| ------------- | ---
| Doc fix?      | no
| New docs?     | yes (symfony/symfony#8133)
| Applies to    | 2.6+
| Fixed tickets | symfony#4082

Commits
-------

286d4f2 describe the allow_extra_fields form option

Author: weaverryan

reference/forms/types/form.rst

@@ -9,6 +9,7 @@ on all types for which ``form`` is the parent type.
 
 +-----------+--------------------------------------------------------------------+
 | Options   | - `action`_                                                        |
+|           | - `allow_extra_fields`_                                            |
 |           | - `by_reference`_                                                  |
 |           | - `cascade_validation`_                                            |
 |           | - `compound`_                                                      |
@@ -52,6 +53,20 @@ Field Options
 
 .. include:: /reference/forms/types/options/action.rst.inc
 
+allow_extra_fields
+~~~~~~~~~~~~~~~~~~
+
+.. versionadded:: 2.6
+    The ``allow_extra_fields`` option was introduced in Symfony 2.6.
+
+**type**: ``boolean`` **default**: ``false``
+
+Usually, if you submit extra fields that aren't configured in your form,
+you'll get a "This form should not contain extra fields." validation error.
+
+You can silence this validation error by enabling the ``allow_extra_fields``
+option on the form.
+
 .. include:: /reference/forms/types/options/by_reference.rst.inc
 
 .. include:: /reference/forms/types/options/cascade_validation.rst.inc