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

[Drew Adams]

Both the Emacs and Elisp manuals describe the behavior of ARG as an
integer or a string.  But what about anything else?

In `startup.el', `edt-mapper.el', and `tpu-edt.el', for example, we find
(kill-emacs t).  How is someone to understand this code?

In GNU Emacs 26.3 (build 1, x86_64-w64-mingw32)
 of 2019-08-29
Repository revision: 96dd0196c28bc36779584e47fffcca433c9309cd
Windowing system distributor `Microsoft Corp.', version 10.0.18362
Configured using:
 `configure --without-dbus --host=x86_64-w64-mingw32
 --without-compress-install 'CFLAGS=-O2 -static -g3''