bug#65027: 30.0.50; [PATCH] Document .elpaignore behavior in the Emacs Lisp manual

[Stefan Monnier]

> Maybe it would make sense to put all the documentation in the Emacs Lisp
> manual, and then the GNU ELPA README can be the home for documentation about
> how to work with the GNU ELPA repository specifically (mainly as an
> administrator).

FWIW, that's how it started.  It's just that some of the conventions
originally used only in (Non)GNU ELPA have now made their way into
`package.el`.

> What do you think?

Any help cleaning this up is welcome.


        Stefan