Add reference to cfg attr

Author: tesuji

src/libstd/macros.rs

@@ -836,9 +836,11 @@ mod builtin {
     /// boolean expression evaluation of configuration flags. This frequently
     /// leads to less duplicated code.
     ///
-    /// The syntax given to this macro is the same syntax as the `cfg`
+    /// The syntax given to this macro is the same syntax as the [`cfg`]
     /// attribute.
     ///
+    /// [`cfg`]: ../reference/conditional-compilation.html#the-cfg-attribute
+    ///
     /// # Examples
     ///
     /// ```