Yes, disabled people do and should have every full right to ask their doctors or caretakers anything they want about their condition. I don't even understand why anyone wouldn't want to fully disclose some information to a patient. If the patient is intellectually disabled, then it is still in their best interest to know what is wrong with them.
People who have disabilities and health issues deserve to know all that they can about them. They should feel free to ask their doctors, and should expect their doctors to take the time to answer the questions. There are some questions no doctors can answer, such as, "How long do I have left?" So we also can't expect them to know everything. But doctors should work to educate themselves on their patients' illnesses and expect to be asked questions.
Disabled people certainly have the right to find out anything they need to know about their condition. If they ask, then they are capable of understanding the answer. Doctors need to be candid if what the person is demanding is candor. They can also demonstrate hopefulness since no one has all the answers on any illness.
The question appears ludicrous from every angle it's viewed upon. We're essentially asking if someone suffering from a medical condition has the right to be fully aware of everything about their medical condition. Of course they should be. There isn't a single culture on earth where this wouldn't be something encouraged.
Disabled people have full right to ask anything they want about their condition. They should be able to learn about anything that affects them if they want to. If they are disabled and want answers then that is their choice. They should be told the truth if they want to hear it.