Document non-interaction of mkRenamedOptionModule and freeformType or attrsOf

[roberth]

          I would guess that `mkRenamedOptionModule` doesn't work because the paths involved aren't actually options. Are you referring to this?

error: evaluation aborted with the following error message: 'cannot find attribute `services.kubo.settings.Experimental.AcceleratedDHTClient''

If it's not a true option, it's not guaranteed to exist, and mkRenamedOptionModule wouldn't be designed to deal with such a situation. Maybe we should have a mkRenamedSettingModule that works for attributes that may not exist? It won't be able to remove the attribute at the old location though (something mkRenamedOptionModule can't do either). So then it may have to be mkRemovedSettingModule? That way we force the user to remove or rename it. It'd be little more than an assertion though.

cc @infinisil

Originally posted by @roberth in #241481 (comment)