Improve Async Docs

[mcmah309]

I was running into issue with async. The current docs are lacking - https://cjycode.com/flutter_rust_bridge/guides/concurrency/async-rust https://cjycode.com/flutter_rust_bridge/guides/concurrency/sync-rust

I had to rummage through the code. Things like flutter_rust_bridge:: exist in examples (https://github.com/fzyzcjy/flutter_rust_bridge/blob/master/frb_example/pure_dart/rust/src/api/async_spawn.rs) but did not work for me, I needed #2151

[fzyzcjy]

Hi, could you please provide a bit more details? For example, when error do you see for a simple async rust function:

async fn f() { }