fixup! Update backport, release and sync documentation

Author: flip1995

book/src/development/infrastructure/release.md

@@ -106,6 +106,9 @@ The `clippy_utils` crate is published to `crates.io` without any stability
 guarantees. To do this, after the [sync] and the release is done, switch back to
 the `upstream/master` branch and publish `clippy_utils`:
 
+> Note: The Rustup PR bumping the nightly and Clippy version **must** be merged
+> before doing this.
+
 ```bash
 git switch master && git pull upstream master
 cargo publish --manifest-path clippy_utils/Cargo.toml