Rollup merge of rust-lang#111973 - Sp00ph:update_current_impl, r=Amanieu

Update current implementation comments for `select_nth_unstable`

This more accurately reflects the actual implementation, as it hasn't been a simple quickselect since rust-lang#106997. While it does say that the current implementation always runs in O(n), I don't think it should require an FCP as it doesn't guarantee linearity in general and only points out that the current implementation is in fact linear.

r? `@Amanieu`

library/core/src/slice/mod.rs

@@ -3005,8 +3005,9 @@ impl<T> [T] {
     ///
     /// # Current implementation
     ///
-    /// The current algorithm is based on the quickselect portion of the same quicksort algorithm
-    /// used for [`sort_unstable`].
+    /// The current algorithm is an introselect implementation based on Pattern Defeating Quicksort, which is also
+    /// the basis for [`sort_unstable`]. The fallback algorithm is Median of Medians using Tukey's Ninther for
+    /// pivot selection, which guarantees linear runtime for all inputs.
     ///
     /// [`sort_unstable`]: slice::sort_unstable
     ///
@@ -3056,8 +3057,9 @@ impl<T> [T] {
     ///
     /// # Current implementation
     ///
-    /// The current algorithm is based on the quickselect portion of the same quicksort algorithm
-    /// used for [`sort_unstable`].
+    /// The current algorithm is an introselect implementation based on Pattern Defeating Quicksort, which is also
+    /// the basis for [`sort_unstable`]. The fallback algorithm is Median of Medians using Tukey's Ninther for
+    /// pivot selection, which guarantees linear runtime for all inputs.
     ///
     /// [`sort_unstable`]: slice::sort_unstable
     ///