Improve documentation explaining passing multiple args in event handlers

[NHDaly]

Explain how to pass an array, and/or use argument destructuring, in order to pass multiple arguments to a handler.

Follow up to #237; fixes #222.

[twavv]

Looks good to me.

Honestly, I think we should structure things a little differently so that we can just "naturally" support multiple arguments. We do this in WebIO with the RPC protocol. We should be able to do that without breaking anything since passing arrays should continue to work.

Alternatively, one thing that I've been liking recently is defining handlers using Val{...} types. I'm not sure it works as well in conjunction with what I described above because it's harder to do with variable numbers of args (without method ambiguity errors). I think it works well for WebIO to define new message types though: https://github.com/JuliaGizmos/WebIO.jl/pull/372/files#diff-f5a345e16e62d96204ee9e29d0386ad4R38

function Blink.handle_msg(::Val{:press}, window::Window, data)
    @show data["foo"]
end

Blink.msg("press", {
    foo: "foo",
});

It's a little bit less dynamic but that might not always be a bad thing (and we could keep the existing "dynamic" method anyway).

[NHDaly]

Yeah that does seem really nice as well! I don't have much preference, but defining functions that are more "normal," definitely seems like an improvement.

For now, I'm going to merge this docs change, since this is the current way to do things, but i'm supportive of moving towards something more like that!