Ballot::getQuestionForm is documented to return a string. Following the subtasks of T273044, it will now return an OOUI FieldsetLayout, which is cast to a string during Ballot::getForm.

We should document that it returns an OOUI FieldsetLayout once these tasks are complete, so that callers know they do not need to treat the return value as a string.

The documentation shouldn't be changed until the subtasks are all complete, because until then, some subclasses still return a string for getQuestionForm.