Help: Add 3.31 release note for file(MAKE_DIRECTORY)'s RESULT option

In commit 95323c90a1 (file(MAKE_DIRECTORY): Add optional RESULT keyword
to capture failure., 2024-06-16, v3.31.0-rc1~414^2) the release note was
added in the wrong directory so it was not gathered for the 3.31 release.

Fixes: #26377

Help/dev/make_directory-optional-result.rst

@@ -1,5 +0,0 @@
-make_directory-optional-result
-------------------------------
-
-* The :command:`file(MAKE_DIRECTORY)` learned to
-optionally capture failure in a result variable.

Help/release/3.31.rst

@@ -59,6 +59,9 @@ Commands
 * The :command:`file(ARCHIVE_CREATE)` command gained a ``WORKING_DIRECTORY``
   option to specify a working directory for the archiving process.
 
+* The :command:`file(MAKE_DIRECTORY)` command gained a ``RESULT`` option
+  to capture failure in a result variable.
+
 * The :command:`install(FILES)` and :command:`install(DIRECTORY)` commands'
   ``TYPE`` argument gained support for a ``LIBEXEC`` type.