bug#12431: [feature request] Add info on next-multiframe-window command

[Eli Zaretskii]

> From: Lars Ingebrigtsen <larsi@gnus.org>
> Date: Fri, 16 Aug 2019 15:31:37 -0700
> Cc: 12431@debbugs.gnu.org
> 
> I've now mentioned that command in the section that talks about
> other-window.

Thanks.

When you mention in a manual something that isn't described in the
same section, please always consider adding a cross-reference to where
that something _is_ described.  We should normally assume that a
subject described in another section is not something the reader is
necessarily well familiar with.