>>> The docstring for outline-minor-mode-use-buttons says its value can be
>>> the symbol insert, but that choice is not present in its :type.
>>>
>>> I can't send a patch, because I can't figure out a good :tag for it.
>>
>> Such omission is intentional, because 'insert' can be used only
>> in generated buffers, not in editable buffers, so disallowing
>> customization to the value 'insert' protects the users
>> from damaging editable buffers.
>
> I see, thanks.  How about adding something to the docstring some text to
> explain that?  And/or maybe add a comment in the :type to clarify that
> the omission is intentional?

Maybe something like this for the emacs-29 branch?