userguide: document exit confirmation dialog

Long description:

In the section "Exiting i3", document the confirmation dialog (nagbar)
that will show when the user presses the default keys to exit in such a
way to hint that it is configurable.

To apply this patch, use:
curl http://cr.i3wm.org/patch/578/raw.patch | git am

@@ -153,6 +153,7 @@ to upgrade to a newer version of i3) you can use +$mod+Shift+r+.

 === Exiting i3

 To cleanly exit i3 without killing your X server, you can use +$mod+Shift+e+.

+By default, a dialog will ask you to confirm if you really want to quit.

 === Floating