Allow async command output buffer to be shown only on output

Allow async command output buffer to be shown only on output

* lisp/simple.el (async-shell-command-display-buffer): Add
defcustom.
(shell-command): Use the new defcustom to determine whether to show
the buffer immediately, or add a process filter that shows it only
when there is some output.
* etc/NEWS: Document the new variable.
* doc/emacs/misc.texi: Likewise.

Thanks to Juri Linkov and Eli Zaretskii for advice and guidance.