bug#67449: Manual: add how to change default global font size in .emacs

[Eli Zaretskii]

> Date: Mon, 27 Nov 2023 04:55:29 +0800
> From: Dan Jacobson <jidanni@jidanni.org>
> Cc: 67449@debbugs.gnu.org
> 
> Fine. If there is a simple way, to change the default font size, one 
> line that one can put in one's .emacs file, without needing to deal with 
> the customization system, then please mention it, right there, below the 
> line that says emacs's default font size is 10. Thank you.

It's already mentioned there.