Improve documentation for UIKit and AppKit handles

[madsmtm]

This takes part of the main thread only docs from #154, but with the less strong guarantee of the handle only ever existing on the main thread (as that's probably not yet true in the wild, so I'd rather have consumers to the extra check).

[Lokathor]

Other than the wasm CI failing, which I think is unrelated, it's always good to have usage example docs.

[madsmtm]

wasm CI failing

Yeah, Winit had the same problem, not sure how cargo hack works so don't know how to fix it, so I'm just gonna merge it regardless.

[Lokathor]

Do we have a "wasm expert" to call for things like this? If
not, should we move the wasm jobs to a separate CI path and just let them fail when they fail?

[madsmtm]

That would be @daxpedda

[daxpedda]

There is no good fix for this, this is a Cargo problem: rust-lang/rfcs#3537.

The best we can do here, which we have done in Winit as well, is pin the bumpalo version in CI:
https://github.com/rust-windowing/winit/blob/a127bd6f6684909852000cdf32469bac3b611887/.github/workflows/ci.yml#L92