add onFocus to the documentation

[NobodyDies]

Here was alot of disscussions about that callback, but for all of cases there was a solution, but i think i found reason why this callback is needed.

In my app i try to make material looking component based on autosuggest, but i cant make it because i can't manipulate animation of label element that is outside of autosuggest based on focus or blur.

here an example of animation that i need to implement https://material.angular.io/components/input/overview

I just do not understand why it is not implemented yet - input changes it class based on focus, so there is some logic for that. I just need to have some ability to tell a wrapper that input is focused or not.

[NobodyDies]

actualy - never mind. Just please - put reference to onFocus callback to the documentation. I found it in the code and used it - working fine