bug#41096: message-sendmail-f-is-evil has hard to understand docstring

[Eli Zaretskii]

> From: Stefan Kangas <stefan@marxist.se>
> Date: Tue, 5 May 2020 20:05:40 +0200
> Cc: 41096@debbugs.gnu.org
> 
> The thing is that I don't understand the docstring as a whole.  The
> first part is fine, but the joke confuses me and makes me wonder what
> I missed.

Here's one attempt to de-confuse it:

     Non-nil means don't add '-f username' to the sendmail command
     line.  Using that option would be even more evil than leaving
     it out.