Update documentation of select_nth_unstable and select_nth_unstable_by and select_nth_unstable_by_key to state O(n log n) worst case complexity

Also remove erronious / in doc comment

Author: schuelermine

library/core/src/slice/mod.rs

@@ -2708,8 +2708,10 @@ 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*) worst-case. This function is also/ known as "kth
-    /// element" in other libraries. It returns a triplet of the following from the reordered slice:
+    /// (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.
+    ///
+    /// It returns a triplet of the following from the reordered slice:
     /// the subslice prior to `index`, the element at `index`, and the subslice after `index`;
     /// accordingly, the values in those two subslices will respectively all be less-than-or-equal-to
     /// and greater-than-or-equal-to the value of the element at `index`.
@@ -2755,8 +2757,11 @@ 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*) worst-case. This function
-    /// is also known as "kth element" in other libraries. It returns a triplet of the following from
+    /// 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.
+    ///
+    /// It returns a triplet of the following from
     /// the slice reordered according to the provided comparator function: the subslice prior to
     /// `index`, the element at `index`, and the subslice after `index`; accordingly, the values in
     /// those two subslices will respectively all be less-than-or-equal-to and greater-than-or-equal-to
@@ -2807,8 +2812,11 @@ 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*) worst-case. This function
-    /// is also known as "kth element" in other libraries. It returns a triplet of the following from
+    /// 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.
+    ///
+    /// It returns a triplet of the following from
     /// the slice reordered according to the provided key extraction function: the subslice prior to
     /// `index`, the element at `index`, and the subslice after `index`; accordingly, the values in
     /// those two subslices will respectively all be less-than-or-equal-to and greater-than-or-equal-to