Add additional documentation to spawn_local

[drewcrawford]

This adds documentation to help troubleshoot why spawn_local might not work as expected.

see #2945 and chemicstry/wasm_thread#6

[daxpedda]

Events, queueMicrotask, setTimeout and Scheduler.postTask() are also affected by this so the issue isn't specific to spawn_local(). I think there needs to be a general understanding that blocking in Wasm is much more often than not a bad idea compared to native.

So its probably better to add documentation to functions that block instead to all possible functions that are affected by blocking. In the case of wasm-bindgen its js_sys::Atomics::wait*. Additionally some top-level documentation somewhere would help make this information be conveyed much more generally.

Thank you for looking into this and going through the effort of a PR!