Apply suggestions from code review

Co-Authored-By: Mazdak Farrokhzad <twingoow@gmail.com>

Author: RalfJung

src/libcore/slice/mod.rs

@@ -293,11 +293,12 @@ impl<T> [T] {
     /// checking.
     ///
     /// This is generally not recommended, use with caution!
-    /// Calling this method with an out-of-bounds index is UB even if the resulting
-    /// reference is not used.
+    /// Calling this method with an out-of-bounds index is *[undefined behavior]*
+    /// even if the resulting reference is not used.
     /// For a safe alternative see [`get`].
     ///
     /// [`get`]: #method.get
+    /// [undefined behavior]: ../../reference/behavior-considered-undefined.html
     ///
     /// # Examples
     ///
@@ -320,11 +321,12 @@ impl<T> [T] {
     /// bounds checking.
     ///
     /// This is generally not recommended, use with caution!
-    /// Calling this method with an out-of-bounds index is UB even if the resulting
-    /// reference is not used.
+    /// Calling this method with an out-of-bounds index is *[undefined behavior]*
+    /// even if the resulting reference is not used.
     /// For a safe alternative see [`get_mut`].
     ///
     /// [`get_mut`]: #method.get_mut
+    /// [undefined behavior]: ../../reference/behavior-considered-undefined.html
     ///
     /// # Examples
     ///
@@ -2633,15 +2635,17 @@ pub trait SliceIndex<T: ?Sized>: private_slice_index::Sealed {
 
     /// Returns a shared reference to the output at this location, without
     /// performing any bounds checking.
-    /// Calling this method with an out-of-bounds index is UB even if the resulting
-    /// reference is not used.
+    /// Calling this method with an out-of-bounds index is *[undefined behavior]*
+    /// even if the resulting reference is not used.
+    /// [undefined behavior]: ../../reference/behavior-considered-undefined.html
     #[unstable(feature = "slice_index_methods", issue = "0")]
     unsafe fn get_unchecked(self, slice: &T) -> &Self::Output;
 
     /// Returns a mutable reference to the output at this location, without
     /// performing any bounds checking.
-    /// Calling this method with an out-of-bounds index is UB even if the resulting
-    /// reference is not used.
+    /// Calling this method with an out-of-bounds index is *[undefined behavior]*
+    /// even if the resulting reference is not used.
+    /// [undefined behavior]: ../../reference/behavior-considered-undefined.html
     #[unstable(feature = "slice_index_methods", issue = "0")]
     unsafe fn get_unchecked_mut(self, slice: &mut T) -> &mut Self::Output;