Cook noodles according to package directions; drain and set aside.Meanwhile, heat oil in a wok or large skillet on medium-high. Add onion and garlic and cook for 2 minutes, until soft, stirring frequently. Add pork and cook for 3 minutes, stirring frequently, until browned on all sides. In a small bowl, whisk together broth and teriyaki sauce. Add broth mixture to wok and bring to a simmer over same heat. Add bok choy, mango and water chestnuts and cook for 1 to 2 minutes, until pork is cooked through and bok choy leaves just begin to wilt (but remain bright green). Remove from heat and season with salt and pepper. Serve pork mixture over noodles.