Added default config for turn username and password

synapse/config/voip.py

@@ -34,6 +34,11 @@ class VoipConfig(Config):
         # The shared secret used to compute passwords for the TURN server
         turn_shared_secret: "YOUR_SHARED_SECRET"
 
+        # The Username and password if the TURN server needs them and
+        # does not use a token
+        #turn_username: "TURNSERVER_USERNAME"
+        #turn_password: "TURNSERVER_PASSWORD"
+
         # How long generated TURN credentials last
         turn_user_lifetime: "1h"
         """