"The Pilgrims" (Credit: PBS) Like the reality of what we would call the first celebration of the holiday, the best Thanksgiving documentary available, Ric Burns’ 2015 PBS film “The Pilgrims,” is cold, deadly and quite a bit more about religion, invasion and chilling tales of hand-to-mouth survival than it is about hearthstone bonhomie between settlers and Native Americans. Baked yams with syrup it’s not. It is, though, a beautiful and necessary exercise in unweaving our calcified national-origin myths — those of the Pilgrims, Thanksgiving and America itself — one that eventually grabs all that loose yarn and with them recreates something whole, revived and, for once, true.