Clarify [T]::select_nth_unstable* return values

In cases where the nth element is not unique within the slice, it is not
correct to say that the values in the returned triplet include ones for
"all elements" less/greater than that at the given index: indeed one (or
more) such values would then laso contain values equal to that at the
given index.

The text proposed here clarifies exactly what is returned, but in so
doing it is also documenting an implementation detail that previously
wasn't detailed: namely that the return slices are slices into the
reordered slice.  I don't think this can be contentious, because the
lifetimes of those returned slices are bound to that of the original
(now reordered) slice—so there really isn't any other reasonable
implementation that could have this behaviour; but nevertheless it's
probably best if @rust-lang/libs-api give it a nod?

Fixes #97982
r? m-ou-se

@rustbot label +A-docs C-bug +T-libs-api

library/core/src/slice/mod.rs

@@ -2628,9 +2628,10 @@ impl<T> [T] {
     /// 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 values: all elements less
-    /// than the one at the given index, the value at the given index, and all elements greater than
-    /// the one at the given index.
+    /// 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`.
     ///
     /// # Current implementation
     ///
@@ -2675,10 +2676,11 @@ impl<T> [T] {
     /// 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
-    /// values: all elements less than the one at the given index, the value at the given index,
-    /// and all elements greater than the one at the given index, using the provided comparator
-    /// 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
+    /// the value of the element at `index`.
     ///
     /// # Current implementation
     ///
@@ -2727,10 +2729,11 @@ impl<T> [T] {
     /// 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
-    /// values: all elements less than the one at the given index, the value at the given index, and
-    /// all elements greater than the one at the given index, using the provided key extraction
-    /// 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
+    /// the value of the element at `index`.
     ///
     /// # Current implementation
     ///