Add unstable feature link and cargo config example

Author: Kobzol

src/doc/src/guide/build-performance.md

@@ -71,18 +71,25 @@ Trade-offs:
 
 ### Enable the experimental parallel frontend
 
-Recommendation: Add `-Zthreads=n` to the `RUSTFLAGS` environment variable[^rustflags], for example:
+Recommendation: Add `-Zthreads=n` to the `RUSTFLAGS` environment variable, for example:
 
 ```console
-$ RUSTFLAGS="-Zthreads=8" cargo build
+$ RUSTFLAGS="-Zthreads=8" cargo +nightly build
 ```
 
-This will enable the [parallel frontend][parallel-frontend] of the Rust compiler, and tell it to use `n` threads. The value of `n` should be chosen according to the number of cores available on your system, although there are diminishing returns. We recommend to set at most `8` threads.
+You can configure `RUSTFLAGS` using [Cargo config](../reference/config.md#buildrustflags), so that you do not have to pass the flag on every cargo invocation manually:
+
+```toml
+# .cargo/config.toml
+[build]
+rustflags = "-Zthreads=8"
+```
+
+This will enable the [parallel frontend][parallel-frontend-blog] of the Rust compiler, and tell it to use `n` threads. The value of `n` should be chosen according to the number of cores available on your system, although there are diminishing returns. We recommend using at most `8` threads.
 
 Trade-offs:
 - ✅ Faster build times
-- ❌ **Requires using nightly Rust and an unstable Cargo feature**
-
-[^rustflags]: You can configure `RUSTFLAGS` either as an environment variable when running Cargo, or through the [Cargo config](../reference/config.md#targettriplerustflags).
+- ❌ **Requires using nightly Rust and an [unstable Rust feature][parallel-frontend-issue]**
 
-[parallel-frontend]: https://blog.rust-lang.org/2023/11/09/parallel-rustc/
+[parallel-frontend-blog]: https://blog.rust-lang.org/2023/11/09/parallel-rustc/
+[parallel-frontend-issue]: https://github.com/rust-lang/rust/issues/113349