Short explanation for & in key ! of dired

[Daniel Clemente]

  Hi, I think that in the docstring of method dired-do-shell-command (assigned 
to key !), it would be useful to add a paragraph like:

---
    Emacs will block until the program ends. To execute the command in the 
background, add `&' to the end of you string, as in `program * &'.
---

  For it's not completely obvious, and it's useful if you're opening graphical 
programs like xpdf, ooffice, a web browser, ...


  Thanks,

Daniel