distance_sensor: update msg template info

msg/distance_sensor.msg

@@ -1,17 +1,17 @@
 # DISTANCE_SENSOR message data
 
-uint32 time_boot_ms
+uint32 time_boot_ms 	# Time since system boot (us)
 
-uint16 min_distance
-uint16 max_distance
-uint16 current_distance
+uint16 min_distance 	# Minimum distance the sensor can measure (cm)
+uint16 max_distance		# Maximum distance the sensor can measure (cm)
+uint16 current_distance	# Current distance reading (cm)
 
-uint8 type
+uint8 type								# Type from MAV_DISTANCE_SENSOR enum
 uint8 MAV_DISTANCE_SENSOR_LASER = 0
 uint8 MAV_DISTANCE_SENSOR_ULTRASOUND = 1
 uint8 MAV_DISTANCE_SENSOR_INFRARED = 2
 
-uint8 id
+uint8 id				# Onboard ID of the sensor
 
-uint8 orientation
-uint8 covariance
+uint8 orientation		# Direction the sensor faces from MAV_SENSOR_ORIENTATION enum
+uint8 covariance		# Measurement covariance (cm), 0 for unknown / invalid readings