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

[Dan Jacobson <jidanni@jidanni.org>]

In (info "(emacs) Fonts") we learn

> By default, Emacs displays text on graphical displays using a 10-point
> monospace font, and the font size can be changed interactively (*note
> Text Scale::).

That is great. But what is 10-point is not the right default size for 
you.

Well, to make a new defaut, just put
(global-text-scale-adjust INCREMENT)
in your .emacs. As simple as that.

But the manual never gets around to mentioning that.

So how shall we fix that problem?

Well first there on In (info "(emacs) Fonts") please add:

> besides interactively, the default can also be incremented in your
> .emacs file: via (global-text-scale-adjust INCREMENT).

OK, also in  (info "(emacs) Text Scale") we see

>  to change the sizes of the fonts globally, type ‘C-x
> C-M-+’, ‘C-x C-M-=’, ‘C-x C-M--’ or ‘C-x C-M-0’, or scroll the mouse
> wheel with both the ‘Ctrl’ and ‘Meta’ modifiers pressed.

That is also great, but you also need to mention how to change it
permanently in .emacs: via (global-text-scale-adjust INCREMENT).

In fact, that would make this index entry
* global-text-scale-adjust: Text Scale. (line  35)
make sense. As currently all it leads to are the keystrokes.

(Note: in all of the above I intentionally do not mention "Easy
customization". That is not what I am trying to document.)

P.S.,
emacs -Q --eval '(global-text-scale-adjust 0)' cause the initial text
to disappear.