season 20 are about to learn some lessons from Tha Doggfather himself!Snoop Dogg is this season's Mega Mentor, making his debut during Monday's Knockout Rounds to help coaches Blake Shelton, Kelly Clarkson, John Legend and Nick Jonas coach their team members up for one last round before the start of the season 20 live shows.«The element of surprise is when you come and see Snoop Dogg sitting in the chair,» the rapper told ET's Kevin Frazier about making his debut as the first-ever rapper to