cmake: Document -E remove_directory symlink behavior

Update documentation for the change made by commit e6c9a8bac3 (cmake:
Teach -E remove_directory to remove directory symlinks, 2019-08-26).
Also add a release note.

Issue: #19533

Help/manual/cmake.1.rst

@@ -536,7 +536,8 @@ Available commands are:
 
 ``remove_directory <dir>...``
   Remove ``<dir>`` directories and their contents.  If a directory does
-  not exist it will be silently ignored.
+  not exist it will be silently ignored.  If ``<dir>`` is a symlink to
+  a directory, just the symlink will be removed.
 
 ``rename <oldname> <newname>``
   Rename a file or directory (on one volume). If file with the ``<newname>`` name

Help/release/dev/remove_directory-symlink.rst

@@ -0,0 +1,6 @@
+remove_directory-symlink
+------------------------
+
+* The :manual:`cmake(1)` ``-E remove_directory`` command-line tool,
+  when given the path to a symlink to a directory, now removes just
+  the symlink.  It no longer removes content of the linked directory.