Help: Add release note about 'copy_directory' behavior change

Issue: #16001

Help/release/dev/copy_directory.rst

@@ -0,0 +1,6 @@
+copy_directory
+--------------
+
+* The :manual:`cmake(1)` ``-E copy_directory`` tool now fails when the
+  source directory does not exist.  Previously it succeeded by creating
+  an empty destination directory.