The medicine needs to be in the original containers with the labels, that's enough to constitute a valid prescription, assuming it's within one year. You don't have to have a separate paper copy of the Prescription.
I'm not aware of any required documentation to bring it onboard.
Maybe what you remember reading about is a form that
DCL has that informs them of any medical needs that a guest has. You can get the form from DCL or if you used a
Travel Agent, they can get it from DCL for you.
After filling out the form, it needs to be faxed back so that DCL can be aware of anything they would have to prepare for.
They have one 'medical needs' form that is used to list anything from a minor problem to a major one. There is a space on there for a signature by a Physician, but it isn't required for every situation.
I used it to inform them of my blood sugar problem that is controlled by meals within a loose schedule and avoiding some foods. I also have asthma, but I don't need a nebulizer. I didn't get the Doctor to sign it, I just submitted it.
Since you'll be bringing your own equipment that requires a Prescription to obtain it, I would suggest getting the medical needs form and faxing it back. If it's under control with the nebulizer and the meds, I wouldn't get the Doctor's signature, unless you think they might have to contact the Doc to discuss any complications before you sail.
hope that helps