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

[Glenn Morris]

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.
Eg completion-list-mode-map.