It means that most doctors are sensible. Some doctors are not sensible. Then either
a) These tend to be 'notable' exceptions as the doctors who become well-known are possibly the more flamboyant/eccentric/odd-thinking/challenging the status quo/fame-seeking/media-loving types, rather than your average GP.
b) Some of the exeptions are quite famous - possibly because they a
Yes; usually, "notable exceptions" indicates that the speaker has some well known exceptions in mind. Since it's a set phrase, though, it's sometimes used where simply "exceptions" would suffice.