bug#42400: 26.3; doc of `kill-emacs' optional ARG

[Drew Adams]

>     Drew> Both the Emacs and Elisp manuals describe the behavior of ARG
> as an
>     Drew> integer or a string.  But what about anything else?
> 
>     Drew> In `startup.el', `edt-mapper.el', and `tpu-edt.el', for
> example, we find
>     Drew> (kill-emacs t).  How is someone to understand this code?
> 
> Itʼs equivalent to exit'ing with EXIT_SUCCESS, without having to worry
> about the value of '0' on your platform :-) Not sure how weʼd express
> that in the docstring.

Maybe just like that?  Is this true only for `t',
or is it true for any non-string, non-number value?

However we say it, I think it should be doc'd.  Thx.