bug#63875: 26.3; Doc of `dired-guess-shell-alist-user'

[Eli Zaretskii]

> From: Drew Adams <drew.adams@oracle.com>
> Date: Sat, 3 Jun 2023 16:08:11 +0000
> 
> The doc string doesn't say what REGEXP is, e.g., what it's intended to
> match.  It should say that up front and explicitly.

???The doc string says:

  Each element of this list looks like

      (REGEXP COMMAND...)

  COMMAND will be used if REGEXP matches the file to be processed.
  If several files are to be processed, REGEXP has to match all the
  files.

"REGEXP matches the file to be processed".  What else needs to be
said?