bug#39233: .elc file - possibly outdated backward compatibility comments

[Stefan Monnier]

> Will these changes still allow old Emacs binaries to load *.elc files
> compiled by Emacs after these changes?

New Emacsen loading old .elc won't be affected.

Old Emacsen loading .elc files compiled by new Emacsen is something that
has always been risky, tho it has sometimes worked.  My proposal is to
make it emit a warning (so it will work as much as it does now, but
will bring to the attention of the user that it's in murky territory).


        Stefan