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.