update AHT20 doc

aht20/doc.go

@@ -3,9 +3,15 @@
 // that can be found in the LICENSE file.
 
 // Package aht20 controls an AHT20 device over I²C.
+//
+// # More details
+//
 // The sensor is a temperature and humidity sensor with a typical accuracy of ±2% RH and ±0.3°C.
 // The aht20.Dev type implements the physic.SenseEnv interface. The physic.Env measurement results
 // contain a temperature, pressure and humidity value though the pressure is not set.
+// The only device address is 0x38.
+//
+// # Datasheet
 //
-// **Datasheet:** http://www.aosong.com/userfiles/files/media/Data%20Sheet%20AHT20.pdf
+// http://www.aosong.com/userfiles/files/media/Data%20Sheet%20AHT20.pdf
 package aht20