Fix usage of `setq-default' and offer more suggestions

Fix usage of `setq-default' and offer more suggestions

cd61af0 changed from default-major-mode to major-mode in the first
code sample but didn't change the rest.  This patch fixes this and add
some explanations of why use `setq-default' instead of `setq'.  In
addition, it gives background on suggesting using text-mode as default
mode and suggest other alternatives.

* doc/lispintro/emacs-lisp-intro.texi (Text and Auto-fill): Fix usage
of `setq-default' and offer more suggestions.  (Bug#67848)

(cherry picked from commit ceacf75395834c452b43932c19df7e3202a16094)