Link “? operator” to relevant chapter in The Book

Before, the text simply asked people to use a symbol which is hard to
search for. Now the text links back to the chapter on error
propagation in The Book. That should help people find the relevant
keywords for further searches.

library/core/src/macros/mod.rs

@@ -350,10 +350,12 @@ macro_rules! matches {
 
 /// Unwraps a result or propagates its error.
 ///
-/// The `?` operator was added to replace `try!` and should be used instead.
-/// Furthermore, `try` is a reserved word in Rust 2018, so if you must use
-/// it, you will need to use the [raw-identifier syntax][ris]: `r#try`.
+/// The [`?` operator][propagating-errors] was added to replace `try!`
+/// and should be used instead. Furthermore, `try` is a reserved word
+/// in Rust 2018, so if you must use it, you will need to use the
+/// [raw-identifier syntax][ris]: `r#try`.
 ///
+/// [propagating-errors]: https://doc.rust-lang.org/book/ch09-02-recoverable-errors-with-result.html#a-shortcut-for-propagating-errors-the--operator
 /// [ris]: https://doc.rust-lang.org/nightly/rust-by-example/compatibility/raw_identifiers.html
 ///
 /// `try!` matches the given [`Result`]. In case of the `Ok` variant, the