The Collaborative International Dictionary of English v.0.48:

Documentary \Doc`u*men"ta*ry\, a.
   Pertaining to written evidence; contained or certified in
   writing. "Documentary evidence." --Macaulay. DoD

WordNet (r) 3.0 (2006):

documentary
    adj 1: relating to or consisting of or derived from documents
           [syn: documentary, documental]
    2: emphasizing or expressing things as perceived without
       distortion of personal feelings, insertion of fictional
       matter, or interpretation; "objective art" [syn: objective,
       documentary]
    n 1: a film or TV program presenting the facts about a person or
         event [syn: documentary, docudrama, documentary film,
         infotainment]

Moby Thesaurus II by Grady Ward, 1.0:

130 Moby Thesaurus words for "documentary":
   3-D, Cinemascope, Cinerama, Technicolor, Western, absolute, actual,
   adducible, admissible, animated cartoon, archival, ascertained,
   attestative, attested, attestive, authentic, authenticated,
   based on, black-and-white film, cartoon, categorically true,
   certain, certified, chiller, cinema, circumstantial, color film,
   conclusive, confirmed, convincing, corroborated, creepie,
   cumulative, damning, decisive, demonstrated, determinative,
   determined, documental, documentary film, documentational,
   documented, educational film, effectual, epigraphic, established,
   evidential, evidentiary, ex parte, eye-witness, factual, feature,
   film, final, firsthand, flick, flicker, founded on, grounded on,
   hearsay, historical, horror picture, horse opera, implicit,
   incontrovertible, indicative, indisputable, irrefutable,
   irresistible, material, motion picture, motion-picture show, movie,
   moving picture, necrological, newsreel, not in error, nudie,
   nuncupative, obituary, objectively true, overwhelming, photodrama,
   photoplay, picture, picture show, pornographic film, presumptive,
   preview, probative, proved, real, reliable, short, significant,
   silent, silent film, skin flick, sneak preview, spaghetti Western,
   substantiated, suggestive, sure, sure-enough, symptomatic, talkie,
   talking picture, telling, testimonial, thriller, trailer, true,
   true as gospel, truthful, unconfuted, undenied, underground film,
   undoubted, unerroneous, unfallacious, unfalse, unmistaken,
   unquestionable, unrefuted, valid, validated, veracious, verified,
   veritable, weighty