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

[Van L]

Hello.

The footnote at line 2150 *indirectly* links to (elisp) Printed Representation, 
which details what an expression is.

: 8cda6f8f44b (Glenn Morris             2007-09-06  2150) learn how to read 
it.@footnote{@code{(quote hello)} is an expansion of

At line 2508 in the summary is an opportunity to *directly* link the mention of 
expressions to (elisp) Printed Representation.

: 8cda6f8f44b (Glenn Morris             2007-09-06  2508) Lisp programs are 
made up of expressions, which are lists or single atoms.

(I will re-open this bug number for future opportunities to link eintr, elisp 
after it is closed.)