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

[Eli Zaretskii]

> From: Fox Kiester <noct@openmailbox.org>
> Date: Wed, 12 Apr 2017 11:40:46 -0400
> 
> 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.

More accurately, this command might decide to silently do nothing if
invoked not vie the menu bar.

I augmented the doc string to say that, thanks.