bug#17567: 24.4.50; doc string of `define-derived-mode'

[Eli Zaretskii]

> From: Stefan Kangas <stefan@marxist.se>
> Date: Sat, 25 Sep 2021 10:15:16 -0700
> Cc: 17567@debbugs.gnu.org
> 
> I have now fixed this on (commit e4c9081d46).  This change will be a
> part of the upcoming Emacs 28.1.

Thanks, but IMO talking about "unquoted symbols" makes this text
confusing.  Why did you need to say that?  My suggestion is to remove
those sentences, they aren't needed.