Add whatsnew note for GH-22904 (#24672)

Author: sweeneyde

Doc/whatsnew/3.10.rst

@@ -615,6 +615,11 @@ Optimizations
   bytecode level.  It is now around 100% faster to create a function with parameter
   annotations.  (Contributed by Yurii Karabas and Inada Naoki in :issue:`42202`)
 
+* Substring search functions such as ``str1 in str2`` and ``str2.find(str1)``
+  now sometimes use Crochemore & Perrin's "Two-Way" string searching
+  algorithm to avoid quadratic behavior on long strings.  (Contributed
+  by Dennis Sweeney in :issue:`41972`)
+
 Deprecated
 ==========