bug#26466: 26.0.50; Document that kill-this-buffer should be called from the menu bar

[Eli Zaretskii]

> From: Glenn Morris <rgm@gnu.org>
> Date: Wed, 12 Apr 2017 15:18:12 -0400
> Cc: 26466@debbugs.gnu.org
> 
> Fox Kiester wrote:
> 
> > I learned today that kill-this-buffer is only meant to be called from
> > the menu bar. This is clear after navigating to its definition in
> > menu-bar.el, but it would be nice if this was also mentioned in its
> > docstring especially since the command is not prefixed with
> > "menu-bar-". This might help prevent some confusion.
> 
> And yet several places call it not from the menu-bar.

Thanks, I made them call another function instead.