msg: Improve vehicle_global_position documentation

Clean up formatting, improve consistency of descriptions and ensure units are defined

msg/vehicle_global_position.msg

@@ -4,18 +4,18 @@
 # estimator, which will take more sources of information into account than just GPS,
 # e.g. control inputs of the vehicle in a Kalman-filter implementation.
 #
-uint64 timestamp		# Time of this estimate, in microseconds since system start
-uint64 time_utc_usec		# GPS UTC timestamp in microseconds
-float64 lat			# Latitude in degrees
-float64 lon			# Longitude in degrees
-float32 alt			# Altitude AMSL in meters
-float32 vel_n			# Ground north velocity, m/s
-float32 vel_e			# Ground east velocity, m/s
-float32 vel_d			# Ground downside velocity, m/s
-float32 yaw 			# Yaw in radians -PI..+PI.
-float32 eph			# Standard deviation of position estimate horizontally
-float32 epv			# Standard deviation of position vertically
-float32 terrain_alt		# Terrain altitude in m, WGS84
+uint64 timestamp		# Time of this estimate since system start, (microseconds)
+uint64 time_utc_usec		# GPS UTC timestamp, (microseconds)
+float64 lat			# Latitude, (degrees)
+float64 lon			# Longitude, (degrees)
+float32 alt			# Altitude AMSL, (meters)
+float32 vel_n			# North velocity in NED earth-fixed frame, (metres/sec)
+float32 vel_e			# East velocity in NED earth-fixed frame, (metres/sec)
+float32 vel_d			# Down velocity in NED earth-fixed frame, (metres/sec)
+float32 yaw 			# Euler yaw angle relative to NED earth-fixed frame, -PI..+PI, (radians)
+float32 eph			# Standard deviation of horizontal position error, (metres)
+float32 epv			# Standard deviation of vertical position error, (metres)
+float32 terrain_alt		# Terrain altitude WGS84, (metres)
 bool terrain_alt_valid		# Terrain altitude estimate is valid
 bool dead_reckoning		# True if this position is estimated through dead-reckoning
-float32 pressure_alt		# Pressure altitude
+float32 pressure_alt		# Pressure altitude AMSL, (metres)