Do the special dance with the destination only if it is a
directory name, for consistency with copy-file etc. (Bug#27986).
* doc/emacs/files.texi (Copying and Naming):
* doc/lispref/files.texi (Create/Delete Dirs):
* etc/NEWS: Document this.
* lisp/files.el (copy-directory): Treat NEWNAME as special
only if it is a directory name.