Here’s what happened when the unexpectedly interim host of NBC’s “Tonight Show” found himself unemployed (albeit with a multimillion-dollar severance) and wittily seething. Filmmaker Rodman Flender’s backstage chronicle of O’Brien’s subsequent musical comedy revue, the “Legally Prohibited from Being Funny on Television Tour,” doesn’t exactly dole out revelations, but what do you want from a vanity project? Somehow Flender, the director of “Leprechaun 2” and lots of TV shows, seems like the right guy to have taken this particular showbiz snaphot. Processing rage and resentment through the usual cerebral sarcasm, O’Brien keeps people at a distance, possibly for their protection and his own. But within the perpetual motion of his manic ambition we glimpse the original Team Coco: the collaborators, family members and in particular one warmly wise assistant who’s always ready to call bullshit when the master compares himself to Mozart, Patton or Anne Frank. Plus: As one of those performers who’s apparently always “on,” even when it’s a nuisance to those around him, O’Brien does at least have the very helpful virtue of genuine hilariousness.