References added

compiler/rustc_error_codes/src/error_codes/E0546.md

@@ -19,9 +19,16 @@ To fix the issue you need to provide the `feature` field.
 #![feature(staged_api)]
 #![stable(since = "1.0.0", feature = "test")]
 
-#[unstable(feature = "unstable_fn", issue = "none")]
+#[unstable(feature = "unstable_fn", issue = "none")] // ok!!
 fn unstable_fn() {}
 
-#[stable(feature = "stable_fn", since = "1.0.0")]
+#[stable(feature = "stable_fn", since = "1.0.0")] // ok!!
 fn stable_fn() {}
 ```
+
+See the [How Rust is Made and “Nightly Rust”][how-rust-made-nightly] appendix
+of the Book and the [Stability attributes][stability-attributes] section of the
+Rustc Dev Guide for more details.
+
+[how-rust-made-nightly]: https://doc.rust-lang.org/book/appendix-07-nightly-rust.html
+[stability-attributes]: https://rustc-dev-guide.rust-lang.org/stability.html