What Makes a Documentary a Documentary?

Waltz with Bashir (Folman 2008) is described as an "animated documentary film" about Ari Folman's time in the Israel Defense Force when he was 19 years old.  Given that 99.5% of what we see is animated, is it accurate to call the film a documentary?  A second related question:  what can the filmmaker achieve using animation vs. something like photographs and archival footage from the events under discussion?