Fix documentation and implementation of 'directory-name-p'

Fix documentation and implementation of 'directory-name-p'

* lisp/files.el (directory-name-p): Modify to recognize
backslashes on MS-Windows and MS-DOS.  Adjust the doc string
accordingly.  Use '=', not char-equal, for comparison, as
letter-case cannot possibly be an issue here.

* doc/lispref/files.texi (Directory Names): Move the documentation
of directory-name-p here from "Relative File Names".  Update the
description per the changes in implementation.

* etc/NEWS: Move the entry for 'directory-name-p' to its proper
place and mark it documented.