[bug#67841] [PATCH] Clarify error messages for misuse of m4_warn and --help for -W.

This was changed in https://debbugs.gnu.org/cgi/bugreport.cgi?bug=67971
and autoconf-2.72 has been released, so closing.

Without Jacob's suggested use of encode(), I see (Jacob, thanks for the
explanations), but Zack, I'm not sure you intended this bug to track
that. Feel free to reopen if so.

Thanks,
Karl