Documentary films aim to document some aspect of reality, typically specifically in areas of instruction or history.
The genre has morphed over time, but recently, they are often used to reveal an unusual, interesting or unknown angle. Topics are limited only by one's imagination as you can see by this huge lists of documentary topics when finding one to watch.
Documentaries, by nature, need to be true. If they're not, they are no longer a documentary film, because they are not documenting anything truthfully. While there are, of course, documentaries that try to lead viewers to see things form their perspective, there still must be some truth to the perspective projected.
Documentary filmmakers are often motivated to make their films because they feel a particular story or viewpoint is not being (adequately) covered by mainstream media.