Add examples to document the return type of select_nth_unstable, `s…

…elect_nth_unstable_by`, and `select_nth_unstable_by_key`.

library/core/src/slice/mod.rs

@@ -3016,8 +3016,13 @@ impl<T> [T] {
     /// ```
     /// let mut v = [-5i32, 4, 2, -3, 1];
     ///
-    /// // Find the median
-    /// v.select_nth_unstable(2);
+    /// // Find the items less than or equal to the median, the median, and greater than or equal to
+    /// // the median.
+    /// let (lesser, median, greater) = v.select_nth_unstable(2);
+    ///
+    /// assert!(lesser == [-3, -5] || lesser == [-5, -3]);
+    /// assert_eq!(median, &mut 1);
+    /// assert!(greater == [4, 2] || greater == [2, 4]);
     ///
     /// // We are only guaranteed the slice will be one of the following, based on the way we sort
     /// // about the specified index.
@@ -3067,8 +3072,13 @@ impl<T> [T] {
     /// ```
     /// let mut v = [-5i32, 4, 2, -3, 1];
     ///
-    /// // Find the median as if the slice were sorted in descending order.
-    /// v.select_nth_unstable_by(2, |a, b| b.cmp(a));
+    /// // Find the items less than or equal to the median, the median, and greater than or equal to
+    /// // the median as if the slice were sorted in descending order.
+    /// let (lesser, median, greater) = v.select_nth_unstable_by(2, |a, b| b.cmp(a));
+    ///
+    /// assert!(lesser == [4, 2] || lesser == [2, 4]);
+    /// assert_eq!(median, &mut 1);
+    /// assert!(greater == [-3, -5] || greater == [-5, -3]);
     ///
     /// // We are only guaranteed the slice will be one of the following, based on the way we sort
     /// // about the specified index.
@@ -3122,8 +3132,13 @@ impl<T> [T] {
     /// ```
     /// let mut v = [-5i32, 4, 1, -3, 2];
     ///
-    /// // Return the median as if the array were sorted according to absolute value.
-    /// v.select_nth_unstable_by_key(2, |a| a.abs());
+    /// // Find the items less than or equal to the median, the median, and greater than or equal to
+    /// // the median as if the slice were sorted according to absolute value.
+    /// let (lesser, median, greater) = v.select_nth_unstable_by_key(2, |a| a.abs());
+    ///
+    /// assert!(lesser == [1, 2] || lesser == [2, 1]);
+    /// assert_eq!(median, &mut -3);
+    /// assert!(greater == [4, -5] || greater == [-5, 4]);
     ///
     /// // We are only guaranteed the slice will be one of the following, based on the way we sort
     /// // about the specified index.