The terminology is confusing. It seems to me that a "feature" film is not necessarily a "non-fiction" film. My sense of "feature" is that it refers more to length than to content. On the other hand, do you ever hear movie biz types or film critics actually talk about "fiction" and "non-fiction" films to begin with? Since 99.999% of all films are "fiction," I guess that's pretty much understood. "Documentary" is a useful term in that it distinguishes a clearly from the rest of the product.
Note that we also don't tend to talk about "fictional" theater pieces, although they too are (largely) works of the imagination.
And we don't tend to interfile dramatic works with fiction in public libraries--although it's all literature. (Academics do it differently, I know.)
I can see a strong argument for including "Woodstock" or "Hoop Dreams" with the so-called "fiction" videos. They're reasonably popular and feature length.