A documentary that advocates a particular, often strong, stance on an issue -- a film that is both a come to Jesus and a call to action.
George: Did you see Gore's 'An Inconvenient Truth'?
Dick: It was amazing. Quite the rantumentary, though it lacked some of the passion of Michael Moore's masterpiece 'Fahrenheit 9/11'.
Short-hand for the word floccinaucinihilipilification, meaning the estimation of something as valueless (encountered mainly as an example of one of the longest words in the English language).