bug#15995: 24.3.50; doc of `append-next-kill': emphasize that `C-M-w' must immediately precede the next kill

[Chong Yidong]

Drew Adams <drew.adams@oracle.com> writes:

> See http://stackoverflow.com/a/20271019/729907.
>
> I was thinking that the doc is already clear enough about this.  But I
> guess I agree with `phils' that it wouldn't hurt to stress that if you
> do something else between `C-M-w' and the kill then the `C-M-w' has no
> effect.

Fixed in trunk, thanks.