Add guide for running doctest (#4393)

Author: Spxg

guide/src/wasm-bindgen-test/usage.md

@@ -127,3 +127,9 @@ Run the tests by passing `--target wasm32-unknown-unknown` to `cargo test`:
 ```
 cargo test --target wasm32-unknown-unknown
 ```
+
+If you also need to run doctests, add the unstable [`-Zdoctest-xcompile`](https://doc.rust-lang.org/nightly/cargo/reference/unstable.html#doctest-xcompile) flag. This requires using the Rust nightly channel like this:
+
+```
+cargo +nightly test --target wasm32-unknown-unknown -Zdoctest-xcompile
+```