Add #[must_use] and update cloned` documentation

Signed-off-by: Connor Tsui <connor.tsui20@gmail.com>
Co-authored-by: Jonas Böttiger <jonasboettiger@icloud.com>

Author: connortsui20

library/core/src/ops/range.rs

@@ -751,6 +751,7 @@ impl<T: Copy> Bound<&T> {
     /// assert_eq!((1..12).start_bound().copied(), Included(1));
     /// ```
     #[unstable(feature = "bound_copied", issue = "145966")]
+    #[must_use]
     pub fn copied(self) -> Bound<T> {
         match self {
             Bound::Unbounded => Bound::Unbounded,
@@ -769,8 +770,11 @@ impl<T: Clone> Bound<&T> {
     /// use std::ops::Bound::*;
     /// use std::ops::RangeBounds;
     ///
-    /// assert_eq!((1..12).start_bound(), Included(&1));
-    /// assert_eq!((1..12).start_bound().cloned(), Included(1));
+    /// let a1 = String::from("a");
+    /// let (a2, a3, a4) = (a1.clone(), a1.clone(), a1.clone());
+    ///
+    /// assert_eq!(Included(&a1), (a2..).start_bound());
+    /// assert_eq!(Included(a3), (a4..).start_bound().cloned());
     /// ```
     #[must_use = "`self` will be dropped if the result is not used"]
     #[stable(feature = "bound_cloned", since = "1.55.0")]