Tardis:Disambiguation states that stories need dab terms at the end. The same does not apply to documentaries. The (documentary) term needs only to be added if there is another article by the same name. (CON episode and DWE episode, etc, are exceptions to this.)
T:DAB TERM lists (documentary) as one of the dab terms which can be used, like (species)--not as an option which must be used, at all times.
If you see documentary titles that have (documentary) unnecessarily, please add {{rename}} or {{speedy rename}}, so that can be rectified.