I just watched this last night and I loved it! I had dismissed it after a friend who worships Bob Kane said it was just hearsay, but then I saw the Penny Arcade comic and here their DLC podcast talking about it, and I had to watch it.
The quick rundown is that Bob Kane had a friend named Bill Finger that was a writer that defined the look of Batman, came up with the backstory for him, came up with all the famous characters and villains (Commissioner Gordon, Robin, Joker, penguin, two face, Scarecrow, etc.), named the city Gotham, and wrote over 1500 of the early Batman comics in the golden age. When Bill first went public with it in the 60's, despite everyone at DC knowing about it, Bob Kane publicly denied it, and continued denying for the rest of Bill's life. You really need to watch the documentary to get the full story here, as it is a good one that gets very emotional at times. So definitely check it out if you have Hulu.