I was wanting to use a clip of Buster Keaton's "The General" in my documentary. I found a clip that is 720p. Does it matter what edition of "The General"? Is there some legal wrangling that I'm not aware of when it comes to editions of the film?

Where did the clip come from? While the movie may be in the public domain, the print may be owned by someone, especially if they did restorative work on it.

Some guy named Charles Cohen, of Cohen Media, bought up the rights to Keaton's films from Raymond Rohauer a few years ago. How does this affect THE GENERAL? I'm not sure.

This is the clip (I would prefer to use it without the music):

<iframe width="420" height="315" src="//www.youtube.com/embed/EEDMO8iwLsM" frameborder="0" allowfullscreen></iframe>

Who are you producing this documentary for? Are you planning to sell it to a broadcaster?

I could be wrong, but I think you are free and clear to use a film that has entered the Public Domain. That includes audio that's just dialogue, but music within the movie may have its own copyright so that also has to be in the Public Domain to use it too. However since you just want the video without the music, I think you're good.

List of Public Domain Films in the U.S.

^ This. You can't take a clip from Kino's Blu-Ray or even a shitty snippet from an Alpha DVD (tho I don't think they'd care).

Try looking at the Internet Archive.