add documentation and link to message definition

ur_msgs/msg/MasterboardDataMsg.msg

+# This data structure contains the MasterboardData structure
+# used by the Universal Robots controller
+#
+# MasterboardData is part of the data structure being send on the 
+# secondary client communications interface
+# 
+# This data structure is send at 10 Hz on TCP port 30002
+# 
+# Dokumentation can be found on the Universal Robots Support Wiki
+# (http://wiki03.lynero.net/Technical/DataStreamFromURController?rev=8)
+
 int16 digital_input_bits
 int16 digital_output_bits
 int8 analog_input_range0

ur_msgs/msg/RobotStateRTMsg.msg

+# Data structure for the realtime communications interface (aka Matlab interface)
+# used by the Universal Robots controller
+# 
+# This data structure is send at 125 Hz on TCP port 30003
+# 
+# Dokumentation can be found on the Universal Robots Support Wiki
+# (http://wiki03.lynero.net/Technical/RealTimeClientInterface?rev=9)
+
 float64 time
 float64[] q_target
 float64[] qd_target