Shortdoc contains a lot of useful examples about how to use common ELisp
functions.  This patch explores a new Emacs feature where those examples
are identified and extracted so they can be displayed, perhaps, in
*Help* buffers (via an additional help-fns-describe-function-functions
hook).

After you install the attached patch, take a look at the *Help* buffer
produced by C-h f assq RET, or C-h f match-string RET, etc.

What do you think of having this new Lisp feature in Emacs 30?  I can
send later another patch with tests, relevant NEWS entries and
documentation changes.

Thanks.