Documenting universal-async-argument

[Michael Albinus]

Hi,

I'd like to document the prefix command universal-async-argument in the
Emacs Lisp manual. But I don't know where.

Section "Prefix Command Arguments" could be a place, but a prefix
command is not a prefix argument. Maybe just a new section after this,
and prior the section "Recursive Editing"? Don't know.

Any hint would be appreciated.

Thanks, and best regards, Michael.