This is only a feature enhancement, not a bug report.

When first learning Emacs, I was often confused about whether a minor mode was enabled globally or just in the current buffer. For example, show-paren-mode is global, but whitespace-mode is buffer-local (unless you use global-whitespace-mode). I still sometimes misinterpret this.

Sure, the user can find out, but the attached patch helps make things clear, by indicating if the mode being toggled is buffer-local. It will be especially helpful for alerting users in the relatively rare cases when they toggle a normally-global minor mode that has been set buffer-locally.

BTW, in define-minor-mode, last-message is indirected, but I don't see any reason not to just use it directly.