Update current impl comment for select_nth_unstable_by_key

Author: Sp00ph

library/core/src/slice/mod.rs

@@ -3113,8 +3113,9 @@ 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
     ///