As far as I can tell, the use of `dired' with an explicit list of file
and directory names is undocumented (beyond the source code). It is
mentioned in neither the Emacs manual nor the Elisp manual.
This is a useful feature - at least as useful as many of the other
Dired features that are documented. Questions about being able to
open Dired with an explicit list of files from different directories
are asked from time to time on help-gnu-emacs.
The logical place to document this is in the Emacs manual, node `Dired
Enter'. That's so, even though to use this feature you must have a
list of file names. A simple example using `defvar' or `setq' to
define such a list would be sufficient - there are similar simple uses
of Emacs Lisp in the Emacs manual.