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

[Drew Adams]

Maybe say something similar to what oantolin says
in this Reddit post:

  "an alist that maps filename regexps to a list
   of commands you might want to apply to the file."

https://www.reddit.com/r/emacs/comments/13yrk5k/is_there_a_package_for_a_opening_a_menu_of_file/jmprpp2/

And maybe make clear that "shell command" here can
also mean any executable program.  (No shell need
be involved.)