bug#31683: emacs-lisp-intro.texi - opportunity to link eintr to elisp

[Van L]

> Eli Zaretskii writes:
> 
> because it is not a reference manual

I was motivated to make this suggestion only because I followed the footnote 
trail to two hops removed and found that snippet worth advertising to a new 
reader a second time incase they didn’t go the distance the first. Anyway. Your 
call.