The documentary aspect wasn't overt. Certainly there were the little bits we got: Londo's narration in "The Gathering" and the first three years of opening narrations (It was the dawn of the... The year the Great War came upon us... The place is Babylon 5. The year is 22XX.); or the character narrations like Ivanova's in "The Fall of Night"; as pointed out, "Deconstruction of Fallen Stars"; the 32 Hours on Babylon 5 news program; and the tag of "Sleeping in Light."
I'd always figured that the series was more of a holographic docudrama, a recreation of the events of 2258-2262 from a variety of sources -- memoirs, journals, logs, ISN newscasts, and documents.
One of the problems with Babylon 5 being a documentary is that it was shot in a very stately manner -- close-ups, two-shots, and wide-shots -- for the most part; a style common in 90s dramas, particularly the SF genre, where the camera is locked down.
Had it been shot in a cinema verite style, then I'd buy the documentary aspect more. Although, you do get a bit of that with the episodes directed by Mike Vejar, who was more experimental in his camera work.