Execute string COMMAND asynchronously in background.
Like `shell-command', but adds `&' at the end of COMMAND
to execute it asynchronously.
The output appears in the buffer `*Async Shell Command*'.
That buffer is in shell mode.
In Elisp, you will often be better served by calling `start-process'
directly, since it offers more control and does not impose the use of a
shell (with its need to quote arguments).