> >> the docstring created by define-minor-mode with nil DOC is useful
> >> and often better than what users write.
> >> Should DOC therefore be optional?
> >
> > Not sure we would like to educate Lisp programmers to stop
> > thinking about good doc strings.
>
> I agree.

Me too.  And definers of user-facing things, such as
defcustom and defface, do require DOC.  Other definers,
such as defconst and defvar, do not require it.

On the other hand, OP raises a real issue, I think.

I'd be in favor of (somehow) automatically having the
definer-provided DOC be augmented by a link that shows
the generic `define-minor-mode' doc, or similar.

IOW:

1. Definers should need to provide a DOC string (even if
   they can fake it with "", which is not encouraged).

2. Users of the mode should have access to the generic
   information also.  It should be sufficient that the
   DOC in the definition provides mode-specific information.
   It should not need to tell users general things about
   using a minor mode.