* lisp/files.el: Use a fixed file name for the second dir-locals file

* lisp/files.el: Use a fixed file name for the second dir-locals file

(dir-locals-file): Revert to its original fixed value.
(dir-locals-file-2): New const.
(dir-locals--all-files): Don't use `file-name-all-completions'.
Instead, just check for the 2 dir-locals files and return a list
of the ones that exit (if any).

* etc/NEWS: Document the change.

* doc/emacs/custom.texi (Directory Variables): Document the change.

* doc/lispref/variables.texi (Directory Local Variables): Update
accordingly.