Is it safe to take Advil (ibuprofen) if I have asthma? I have taken it before without adverse reactions, but the bottle cautions asthma users to consult a doctor

Possibly. Aspirin exacerbated respiratory disease is a form of asthma that is worsened when one is exposed to Aspirin and Aspirin like medications. One's physician should be able to clarify if one has this type of asthma.
Advil (ibuprofen) with asthma. If you have taken it before and had no problem and if you are not allergic to Aspirin then you can safely take it. Asmall number of patient with asthma also have Aspirin allergy and that can cross over to advil and Aleve (naproxen) etc, but number is very small.