Help: Add 3.27 release note for TARGET_RUNTIME_DLL_DIRS

This was accidentally left out of commit 2ce3d62ffb (Help: add
documentation for the new TARGET_RUNTIME_DLL_DIRS genex, 2023-02-26,
v3.27.0-rc1~400^2~2).

Help/release/3.27.rst

@@ -157,6 +157,10 @@ Generator Expressions
   :ref:`transformation <GenEx Path Transformations>` operations learned
   to process :ref:`lists <CMake Language Lists>` of paths element-wise.
 
+* The :genex:`TARGET_RUNTIME_DLL_DIRS` generator expression was added.
+  It expands to a list of the directories containing DLLs in
+  :genex:`TARGET_RUNTIME_DLLS`.
+
 Autogen
 -------