Link the reference about undefined behavior

Suggested-by: Mara Bos <m-ou.se@m-ou.se>
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>

Author: ojeda

library/core/src/option.rs

@@ -433,7 +433,9 @@ impl<T> Option<T> {
     ///
     /// # Safety
     ///
-    /// Undefined behavior if the value is [`None`].
+    /// Calling this method on [`None`] is *[undefined behavior]*.
+    ///
+    /// [undefined behavior]: https://doc.rust-lang.org/reference/behavior-considered-undefined.html
     ///
     /// # Examples
     ///

library/core/src/result.rs

@@ -827,7 +827,9 @@ impl<T, E> Result<T, E> {
     ///
     /// # Safety
     ///
-    /// Undefined behavior if the value is an [`Err`].
+    /// Calling this method on an [`Err`] is *[undefined behavior]*.
+    ///
+    /// [undefined behavior]: https://doc.rust-lang.org/reference/behavior-considered-undefined.html
     ///
     /// # Examples
     ///
@@ -859,7 +861,9 @@ impl<T, E> Result<T, E> {
     ///
     /// # Safety
     ///
-    /// Undefined behavior if the value is an [`Ok`].
+    /// Calling this method on an [`Ok`] is *[undefined behavior]*.
+    ///
+    /// [undefined behavior]: https://doc.rust-lang.org/reference/behavior-considered-undefined.html
     ///
     /// # Examples
     ///