Document the _js parameter in events

[freddyaboulton]

• I have searched to see if a similar issue already exists.

Is your feature request related to a problem? Please describe.
As a gradio user, the _js parameter for event updates is not documented in the website. It is not clear to me what it's for and how to use it.

Describe the solution you'd like
I think we should write a guide on how to use the _js parameter in practice. It's used under the hood by interfaces in the reset button as well as in the block_js_methods demo so those can be motivating examples.

Additional context
Similar to #1368 but I argue it's different. #1368 talks about documenting the event signatures in the Docs page. I think we should go into how _js works via examples/demos.