Azad, there are documentary screenplays for the kind of documentaries you might see on Discovery Channel or National Geographic – a science or history program, for example, where they basically know what they want to say and show and create the visuals around the script. But I think what Doug is referring to are more what we would call "creative documentaries." Certainly documentaries which rely heavily on observational or verite footage cannot be scripted in advance. But some filmmakers do find it helpful to put a film in script form once they have the footage as a means of organizing the information as they edit. Others work without a script and use other systems to organize the film.
While there is no rule in documentary of how to create this script that would correspond to Hollywood's rules of how to create a fiction screenplay, there are some rules of thumbs you might want to go by. I find it more helpful to use the 2 column approach with video on the left and audio on the right (possibly with a third column to denote tape info and timecode in/outs if you are actually editing to the script), though I have seen documentary scripts which look like fiction scripts too.
But to answer your original question which I think is really "How can I write down an idea of what my film will be when I don't know who my characters are yet or what will transpire?" that is really more what a treatment will achieve. Again, there is no right way of doing a treatment (though different funders may have specific elements they want in the treatment) and it will definitely change as you discover your characters and story. For now, it may suffice to write down what it is you want to say with your film, set the stage of where it is taking place, the kinds of characters and/or events you will be seeking out, and why you are the one to tell this story.