minor #4152 Adding 'attr' option to the Textarea options list (ronanguilloux)

This PR was submitted for the 2.5 branch but it was merged into the 2.3 branch instead (closes #4152).

Discussion
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Adding 'attr' option to the Textarea options list

| Q             | A
| ------------- | ---
| Doc fix?      | yes
| New docs?  | nope
| Applies to    | all?
| Fixed tickets | ?

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

Commits
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d6f9bd1 Adding 'attr' option to the Textarea options list

Author: weaverryan

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