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

[Dan Jacobson]

So the default size "10" must be buried in the C source code, and if
the user wants to change *it* they must recompile emacs.
Too bad.

P.S.,
$ emacs -Q -eval "(set-face-attribute 'default nil :height (* 18 6))"
tests great! (Except I found I needed to multiply any value I wanted by
six, in order to get that value. At least on a Chromebook.)