Not a documentary, but if you want some good reading in comic book form, I highly recommend the following:
Third one comes out soon.
Source: Hip Hop Family Tee by Ed Piskor. A really cool series if you're interested in the origins of hip hop and rap. There's a bunch of them online at boing boing, or they've been published in book form by Fantagraphics, or of course you can get them from Amazon.
The comic can be bought on Amazon or Fantagraphics Book's website.
Hip Hop Family Tree by Ed Piskor is fantastic, it's a comic book but it's non fiction and filled with information. There's 3 volumes so far detailing the birth of hip hop, starting in the 70's going into the mid 80's. He's actively writing the next volumes and I can't wait for him to get to the late 80's, 90's.
Hip Hop Family Tree Vol. 1 (70's - '81)