bug#3577: 23.0.94; Info manual does not explain MTIME argument for dir-locals-set-directory-class

Info nodes

    (emacs) Directory Variables
    (elisp) Directory Local Variables

explain or give examples of function dir-locals-set-directory-class but
neither of them mentions the mandatory third argument MTIME. I quote the
doc string of the function:

    dir-locals-set-directory-class is a compiled Lisp function in
    `files.el'.

    (dir-locals-set-directory-class directory class mtime)

    Declare that the directory root is an instance of class.
    directory is the name of a directory, a string.
    class is the name of a project class, a symbol.
    mtime is either the modification time of the directory-local
    variables file that defined this this class, or nil.

    When a file beneath directory is visited, the mode-specific
    variables from class are applied to the buffer.  The variables
    for a class are defined using `dir-locals-set-class-variables'.