>>> No, I want the default Isearch to fold `' with the fancy variants in
>>> these modes.  :-)
>>
>> So the users will have no way to search for ' literally?
>
> Yes, I don't think anybody would want to.  But like I said, I'd prefer
> if it was possible to have this as a minor mode so that people could
> switch it off (even if they probably won't want to).

If you are sure about this, here is the simplest way to implement it,
and the users can disable it easily in help-mode-hook: