not really a documentary, but I watched the movie "As Dreamers Do" on Friday which is a about about Walt's days before he moved to CA (as kid, his time with the Red Cross in France, Laugh-O-Gram studios) ...unfortunately if you haven't been following along with the production of the movie and pre-ordered it chances are you prob won't be able to see it for a few months as they are actively trying to shop it to film festivals and theaters to screen it
All that said, I really liked...the kid acting is kinda lame but I thought all the adults in it did a great job, especially the guys that played Elias, Walt, and Ub Iwerks, and Travis Tritt is the (off-screen) narrator and does one of the songs...since this is a fictionalized, unofficial version of his childhood so if you expect a true history lesson this isn't it, but there is enough there to get a sense of who Walt was as a child...the writer and director did a good job of inserting some little inside jokes for the fans too...I think it was well worth my $ and will pay to see it on the big screen if they screen it in Orlando...go check out there web site and Facebook page "As Dreams Do" for more info or @waltmovie on twitter
(and though some of that sounds like a commercial, I am in no way connected to the movie)