My two cents... as far as subject matter is concerned, documentaries
are secondary resources and should be treated like all other secondary
resources: books, journal articles, etc. One should not think of the
documentary film as a primary historical resource just because it
seems somehow more "direct". All documentaries are subjective.
HOWEVER, documentaries ARE a primary source in one respect. They
record the dynamic processes of reportage, storytelling, sense-making,
etc. It's important to study not just the content of documentaries,
but also how documentaries express that content, and how these
expressions are fundamentally linked to the socio-political conditions
in which they were formed.