Update documentation on extern C async

[matthiasgeihs]

Motivation

Was getting some unexpected behavior when working with extern "C" in combination with async.

#[wasm_bindgen]
extern "C" {
    async fn f() -> String;
}

This does not compile.

mismatched types
expected struct `std::string::String`, found struct `wasm_bindgen::JsValue`

Currently it seems like we cannot have return type different than JsValue. (Implementing From<JsValue> doesn't help.) However, there appears to be no information on this in the official documentation.

After some searching, I found some background info on this in the PR implementing the functionality: #2196 (comment). Basically it seems that only return types JsValue is supported.

Proposed Solution

Add relevant information about allowed return types in async extern C to official documentation.

(Might create a PR for this myself if I find time, but wanted to document here now.)

[matthiasgeihs]

My bad: The documentation actually mentions that the return type must be JsValue or ().

https://rustwasm.github.io/wasm-bindgen/reference/js-promises-and-rust-futures.html