I've seen interesting documentaries on sasquatch sightings. Area 51 and perpetual motion machines. :-)
I said primary source - a documentary about area 51 is not a primary source about area 51. It is a primary source about the documentary itself, however, and in that respect is completely reliable. If the documentary says "XYZ" then you can be absolutely sure the documentary says "XYZ" - reliability is generally trivial for primary sources.