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

[Drew Adams]

> Feel free to close the bug.  Thx.

But it would be good, I think, to add that it's
not just about shell commands.  Some users will
not recognize that they can invoke executable
programs this way.

E.g., on MS Windows, users who never use anything
like a shell explicitly will nevertheless want to
know how to associate files (by name matches) with
applications, in Emacs.