Kuleshov did some experiments with how much power editing has over the outcome of the film. He took an actor and filmed that actor with just an expressionless face. Than he took that shot of an expressionless face, and combined it with a few different shots. One shot was of food, one was of a little girl playing cubes, and another one of an old and tired woman. When the audience were shown the face after a shot of food, they felt the person looked hungry. Than it was combined with a scene of a little girl playing cubes. The audience felt the face showed happiness. Then it was combined with a scene of an old woman. The audience felt the face was showing grief.
So, essentially, the way you combine shots in the editing room, can give the same shots a completely different feel. Here's another example.
Create two scenes. One is were a boy is swimming peacefully at a lake. Another one is a scene of a shark in Africa with scary jaws and fins. Depending on how the editor combines those two scenes, he can create an horror film similar to Jaws, create a comedy film, or create a documentary. Add some music to it, and you can create whatever feeling you want.
That is why an editor is such a crucial member of the film team. A film can literally be rewritten in the editing stage.
Also, if you are dreaming about directing for a major Hollywood studio, be ready to have no control over the editing. In that world, editing is decided by the producers, and only a few directors have the privilege of having any say in the final product. That's why there's a thing called the Director's Cut, and there's a cut that is actually shown to the audiences.
Also, documentaries are really not about presenting facts. They are about presenting facts that support the producer's point-of-view on the subject. Ever heard of people who gave an interview for a documentary or a news station, just to find out how it was distorted when it was shown? That's the power of editing.