bug#49420: 28.0.50; Add elisp manual entry for subst-char-in-string

[Eli Zaretskii]

> From: Lars Ingebrigtsen <larsi@gnus.org>
> Cc: Eli Zaretskii <eliz@gnu.org>,  49420@debbugs.gnu.org
> Date: Sat, 10 Jul 2021 19:00:24 +0200
> 
> Filipp Gunbin <fgunbin@fastmail.fm> writes:
> 
> > Something like this?
> 
> Thanks; applied to Emacs 28.

Thanks.  I made a minor fix in this change: the part about both
characters having to have the same multibyte length is incorrect (that
restriction is only applicable to subst-char-in-region).  I also fixed
the sentence with double negation ("Unless ... non-nil, ...".)

Sorry I missed the original patch.