Documentaries can be a beast, they can encompass hundreds if not thousands of hours of source footage. They require a bit of a different approach, and the gentleman in this seminar goes over quite a bit of good info to know about when getting yourself into a documentary.
One of the main advantages of doing a documentary is also one of the most daunting: you can use just about any source footage you want. Content is king in docs, no one really cares about 4k, jib shots, etc..., and since there's no 4th wall to hide behind, you can use the nastiest, grittiest, most degraded footage you like, so long as it is a meaningful addition to the story. That means there is potentially millions of hours of footage out there, and you need to find the bits and pieces that would best tell the story of your doc. That isn't to say that documentaries are made up entirely of stock/source footage, not at all, many docs use mostly original footage by the filmmakers, but the key here is that they/you are not by any means restricted to using just footage you shoot.
Such a task requires lots of planning, scripting a story to tell, workflow, data management, coffee or caffeinated tea, and patience. And all of it culminates to what happens in the editing room. Lack of planning at the start, will only make the editing more difficult.
So grab some of that coffee/tea and see what this seminar has to offer on preparing you to edit your next/first documentary.