New user option checkdoc-allow-quoting-nil-and-t

New user option checkdoc-allow-quoting-nil-and-t

* lisp/emacs-lisp/checkdoc.el (checkdoc-allow-quoting-nil-and-t):
New user option that allows turning off the warning for having nil
and t in quotes.
(checkdoc-this-string-valid-engine): Use above new option.

(cherry picked from commit 8d4feb214b7eb802fec34269de11d30b9382cd83)