rpcserver+docs: add default case to addr type switch

This commit adds a default case to the address type switch statement in
the NewAddress rpc server function. This catches any invalid address
types and returns an error.

rpcserver.go

@@ -1542,6 +1542,9 @@ func (r *rpcServer) NewAddress(ctx context.Context,
 		if err != nil {
 			return nil, err
 		}
+
+	default:
+		return nil, fmt.Errorf("unknown address type: %v", in.Type)
 	}
 
 	rpcsLog.Debugf("[newaddress] account=%v type=%v addr=%v", account,