How might documentary films blur the distinction between objectivity and subjectivity that we associate with the label nonfiction? How do they sometimes "teach" something more than just the facts?

Documentary films are short films made as per the facts that exits regarding an incident, an event or a theory that exists.

As compared to the mainstream films that are majorly made out of one's imagination and expression of feelings, documentaries are made to explore the already existing facts that may have been unexplored before or may want to add more perspective to the already known facts.

Explanation:

Keeping the above in mind, documentary films specifically are categorized as non-fictional films, but at the same time present more than just the facts or the objectivity of the issue it revolves around.

It at times tries to persuade or question the believes prevailing among the masses, or even argues about the facts that are so far known.

This way, the approach it uses makes the objective and the subjective idea of "nonfiction" a miss. Through its way, it subtly gives more information and conveys its theory to the masses without expressing more.