Rollup merge of rust-lang#119481 - romanows:fix-doc-select-nth-unstable, r=Mark-Simulacrum

Clarify ambiguity in select_nth_unstable docs

Original docs for `select_nth_unstable` family of functions were ambiguous as to whether "the element at `index`" was the element at `index` before the function reordered the elements or after the function reordered the elements.

The most helpful change in this PR is to change the given examples to make this absolutely clear.  Before, "the element at `index`" was the same value before and after the reordering, so it didn't help disambiguate the meaning.  I've changed the example for `select_nth_unstable` and `select_nth_unstable_by` so that "the element at `index`" is different before and after the reordering, which clears up the ambiguity.  The function `select_nth_unstable_by_key` already had an example that was unambiguous.

In an attempt to clear up the ambiguity from the get-go, I've added a bit of redundancy to the text.  Now the docs refer to "the element at `index` *after the reordering*".

Author: matthiaskrgr

library/core/src/slice/mod.rs

@@ -2989,7 +2989,7 @@ impl<T> [T] {
         sort::quicksort(self, |a, b| f(a).lt(&f(b)));
     }
 
-    /// Reorder the slice such that the element at `index` is at its final sorted position.
+    /// Reorder the slice such that the element at `index` after the reordering is at its final sorted position.
     ///
     /// This reordering has the additional property that any value at position `i < index` will be
     /// less than or equal to any value at a position `j > index`. Additionally, this reordering is
@@ -3017,7 +3017,7 @@ impl<T> [T] {
     /// # Examples
     ///
     /// ```
-    /// let mut v = [-5i32, 4, 1, -3, 2];
+    /// let mut v = [-5i32, 4, 2, -3, 1];
     ///
     /// // Find the median
     /// v.select_nth_unstable(2);
@@ -3038,8 +3038,8 @@ impl<T> [T] {
         select::partition_at_index(self, index, T::lt)
     }
 
-    /// Reorder the slice with a comparator function such that the element at `index` is at its
-    /// final sorted position.
+    /// Reorder the slice with a comparator function such that the element at `index` after the reordering is at
+    /// its final sorted position.
     ///
     /// This reordering has the additional property that any value at position `i < index` will be
     /// less than or equal to any value at a position `j > index` using the comparator function.
@@ -3068,7 +3068,7 @@ impl<T> [T] {
     /// # Examples
     ///
     /// ```
-    /// let mut v = [-5i32, 4, 1, -3, 2];
+    /// 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));
@@ -3093,8 +3093,8 @@ impl<T> [T] {
         select::partition_at_index(self, index, |a: &T, b: &T| compare(a, b) == Less)
     }
 
-    /// Reorder the slice with a key extraction function such that the element at `index` is at its
-    /// final sorted position.
+    /// Reorder the slice with a key extraction function such that the element at `index` after the reordering is
+    /// at its final sorted position.
     ///
     /// This reordering has the additional property that any value at position `i < index` will be
     /// less than or equal to any value at a position `j > index` using the key extraction function.