Add a minor clarification

Hopefully this will save the next person from wasting an hour+ on this.

src/doc/src/reference/build-scripts.md

@@ -444,7 +444,7 @@ prevent running the build script in question altogether and instead supply the
 metadata ahead of time.
 
 To override a build script, place the following configuration in any acceptable
-Cargo [configuration location](config.md).
+Cargo [configuration location](config.md) (e.g. `config.toml`, _not_ `Cargo.toml`).
 
 ```toml
 [target.x86_64-unknown-linux-gnu.foo]