[DOC] Add action docs to input and textarea helpers

I could not find any doc to explain how input helpers can send actions. I am not sure it has been forgotten or it was intentionally done.

I think, there are still some steps to be completed:

- [x] Add documentation to `TextSupport` properties of events (`focus-in`, `focus-out`...) like `action`.
- [x] Add missing tests of the different actionable events.


I do not think this actually gets sent to the `controller`, I believe it is sent to the component instance (as `Ember.TextField` and `Ember.TextArea` are components).