bug#58843: Print "decrypted" rot13 text is buffer is read-only

[Eli Zaretskii]

> From: Philip Kaludercic <philipk@posteo.net>
> Cc: 58843@debbugs.gnu.org
> Date: Sat, 29 Oct 2022 15:32:10 +0000
> 
> > Sure, that would be even better.  Although in that case we'd need to
> > change what the manual says about the command, because it explicitly
> > says we show the encoded text in another window.
> 
> How is this:

LGTM, but what about the suitable change to the user manual?

Thanks.