Mattias, thanks for going through the Emacs manual and looking for mistakes in 
this area. I know it was a pain to do that, since I did something similar in 
parallel and it was painful for me. I used your patch to crosscheck with my 
draft (finding omissions on both sides) and installed the resulting patch 
(attached) into the emacs-27 branch.

This patch should address the points that Eli raised. That is, it adds 
explanations of the issue (both in the intro and the reference manual, since the 
issue also infects the intro), and it attempts to change examples only when the 
changes are needed to avoid undefined behavior in Emacs Lisp. I also kept the 
changes from '< to #'< that were in your patch since that's good style.