Conditioning means the gradual presentation of information in order to reinforce its perception as something true.
The more something's repeated,
the more real it seems.
The most basic form of mind-control is repetition. It works.
So, considering documentary filmmaking in the 21st Century, this can be applied in many ways.
Within a film, a core idea could be repeated & varied endlessly.
This will drill into the audience's subconscious the "truth" of the matter at hand.
For example, the ideas within the documentary "What the Bleep do we Know?" create an odd sensation of truth by the end of the film.
This occurs through multiple types of conditioning:
One, because so many different types of people gradually relay the agenda of the movie.
And two, because one step at a time, little by little, the subjects and narrator slowly increase the incredulity of the claims, to the point where by the end the most outlandish ideas appear, at least at the moment, credible.
Or, in the case of a subscription-based online channel, the daily / weekly continuity of your message will significantly reinforce in people's minds the concreteness of your ideas.
This is about documentaries.
This is about information.
Take such documentary-personalities like Alex Jones, for example.
Or former presidential candidate Donald Trump.
The repetition of their message, the gradual presentation of their presence - over time - creates a feeling that they belong in the cultural sphere.
Okay, but let's assume you're not trying to create some cockamaney or evil information stream.
How does this idea apply to the average documentary?
The same applies.
"An Inconvenient Truth" "Kurt & Courtney" "Bitter Lake" ... all of these movies present an overarching theory of how the information within the movie should be digested.
Many of the feelings of legitimacy for your conclusions will be predominantly derived from the concept of audience conditioning.
From repetition, repetition, repetition.
The gradual presentation of information will alter an audience's perception over time.