docs: update docs for focus change event (#784)

[illiteratewriter]

I was also thinking of adding the following lines to the example in the docs:

execute!(
        stdout,
        EnableBracketedPaste,
        EnableFocusChange,
        EnableMouseCapture,
    )?;

so that there is better clarity in the example. Let me know if this sounds like a good addition.

[joshka]

I was also thinking of adding the following lines to the example in the docs:

Yes - this would be a good idea. Don't forget the corresponding disable though.