ON January 21, public television stations across the country will air Near Death, a documentary on decisions about life-sustaining treatment. This six-hour film, the latest work of Frederick Wiseman, is an immersion in the daily life of the medical intensive care unit at Boston's Beth Israel Hospital. It performs a great service, revealing a reality many laypeople and doctors are sure to face. The film provides no overarching commentary, however, to put into perspective the practices it depicts. It falls to the viewer to deduce the strengths and weaknesses of those practices and to glean the lessons of Near Death.