Better document limitations of file pickers in liveview

## Feature Request

I suspect the file picker in liveview is broken:

* The dialog box to choose a file did not show up, I had to inject some JS to disable the event propagation to the server for onclick.

* The FileEngine object was not returned, I had to inject some JS to download the file.

I realize liveview is going to be refactored soon, just wanted to report what might be a potential frustration for new users :)

## Implement Suggestion

Update docs