bug#38587: base64-decode-region breaks encoding

[Eli Zaretskii]

> From: Lars Ingebrigtsen <larsi@gnus.org>
> Date: Fri, 13 Dec 2019 03:52:46 +0100
> Cc: 38587@debbugs.gnu.org
> 
> > (base64-decode-string (base64-encode-string "ä"))
> > => "\344"
> 
> Well, that's not really reversible, either.
> 
> > (the latter expression returns the right result, but inserts broken text 
> > too)
> 
> None of these functions work on multibyte text (by design)

Right.

> but I see the doc strings don't mention this.  (The manual does.)

Let's say that in the doc strings as well.  It is not easy to come up
with the right text, btw, because saying just "region must be unibyte"
is inaccurate; see the source of the implementation for the details.
That's why the ELisp manual also doesn't say anything simple in this
respect.