A documentary is defined as a movie which uses "pictures or interviews with people involved in real events to provide a factual report on a particular subject."
Facts, then, are an important element in a documentary.
Facts are something to consider.
A fact is "a thing that is indisputably the case."
Common sense would then reason that a documentary should not solely present facts, but interesting facts.
What value would a documentary have in telling what's already known?
The job of the documentarian, then, is not just delivering a factual report - but an INTERESTING factual report.
In its simplest form, that means NEW FACTS.
But this could also mean other things.
This means you'll have to do a bit more research, but that's okay.
Research is rad.
You get to drink coffee and read old books all afternoon.
You'll be fine.
If what you're describing is already common knowledge, leave it on the cutting room floor.