Rollup merge of #111974 - Sp00ph:update_guarantees, r=Amanieu

Update runtime guarantee for `select_nth_unstable`

#106933 changed the runtime guarantee for `select_nth_unstable` from O(n) to O(n log n), since the old guarantee wasn't actually met by the implementation at the time. Now with #107522, `select_nth_unstable` should be truly linear in runtime, so we can revert its runtime guarantee to O(n). Since #106933 was considered a bug fix, this will probably need an FCP because it counts as a new API guarantee.

r? `@Amanieu`

library/core/src/slice/mod.rs

@@ -2995,7 +2995,7 @@ impl<T> [T] {
  /// 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
  /// unstable (i.e. any number of equal elements may end up at position `index`), in-place
- /// (i.e. does not allocate), and *O*(*n*) on average. The worst-case performance is *O*(*n* log *n*).
+ /// (i.e. does not allocate), and runs in *O*(*n*) time.
  /// This function is also known as "kth element" in other libraries.
  ///
  /// It returns a triplet of the following from the reordered slice:
@@ -3045,9 +3045,8 @@ impl<T> [T] {
  /// 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.
  /// Additionally, this reordering is unstable (i.e. any number of equal elements may end up at
- /// position `index`), in-place (i.e. does not allocate), and *O*(*n*) on average.
- /// The worst-case performance is *O*(*n* log *n*). This function is also known as
- /// "kth element" in other libraries.
+ /// position `index`), in-place (i.e. does not allocate), and runs in *O*(*n*) time.
+ /// This function is also known as "kth element" in other libraries.
  ///
  /// It returns a triplet of the following from
  /// the slice reordered according to the provided comparator function: the subslice prior to
@@ -3101,8 +3100,7 @@ impl<T> [T] {
  /// 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.
  /// Additionally, this reordering is unstable (i.e. any number of equal elements may end up at
- /// position `index`), in-place (i.e. does not allocate), and *O*(*n*) on average.
- /// The worst-case performance is *O*(*n* log *n*).
+ /// position `index`), in-place (i.e. does not allocate), and runs in *O*(*n*) time.
  /// This function is also known as "kth element" in other libraries.
  ///
  /// It returns a triplet of the following from