The backups are stepping up. The reserves are more than filling in. Boston’s second unit is playing as well — if not better, in some cases — than the first. But Doc Rivers wants it known that the Celtics’ bench, which produced 52 points in a 106-104 win against the Clippers on Sunday at the Garden, didn’t just start playing well. This has been an ongoing trend, he noted, that has just now picked up notice.