Adding 'attr' option to the Textarea options list

`attr` is a quite useful option, but not yet listed here. BTW, the `attr.rst.inc` file already includes an `attr` conf. example concerning textareas.

Author: ronanguilloux

reference/forms/types/textarea.rst

@@ -9,8 +9,9 @@ Renders a ``textarea`` HTML element.
 +-------------+------------------------------------------------------------------------+
 | Rendered as | ``textarea`` tag |
 +-------------+------------------------------------------------------------------------+
-| Inherited | - `data`_ |
-| options | - `disabled`_ |
+| Inherited | - `attr`_ |
+| options | - `data`_ |
+| | - `disabled`_ |
 | | - `empty_data`_ |
 | | - `error_bubbling`_ |
 | | - `error_mapping`_ |
@@ -32,6 +33,8 @@ Inherited Options
 
 These options inherit from the :doc:`form </reference/forms/types/form>` type:
 
+.. include:: /reference/forms/types/options/attr.rst.inc
+
 .. include:: /reference/forms/types/options/data.rst.inc
 
 .. include:: /reference/forms/types/options/disabled.rst.inc