bug#2433: 23.0.90; [DOC] Sec.21, Command Loop, does not describe undo-related behaviour.

[Chong Yidong]

> Section 32.9 "(Elisp)Undo" briefly describes that the command loop
> creates an undo-boundary before a key sequence is executed.
>
> Section 31 "(Elisp)Command Loop" does, however, not elaborate on
> that. In fact, it does not mention undo-related behaviour at all.  In
> particular, it's not mentioned where exactly the undo-boundary is
> pushed onto the undo-list.

Fixed, thanks.