[AIRFLOW-2959] Refine HTTPSensor doc

HTTP Error code other than 404,
or Connection Refused, would fail the sensor
itself directly (no more poking).

This is not clear enough in the documentation.

airflow/sensors/http_sensor.py

@@ -28,8 +28,11 @@
 
 class HttpSensor(BaseSensorOperator):
     """
-    Executes a HTTP get statement and returns False on failure:
-        404 not found or response_check function returned False
+    Executes a HTTP GET statement and returns False on failure caused by
+    404 Not Found or `response_check` returning False.
+
+    HTTP Error codes other than 404 (like 403) or Connection Refused Error
+    would fail the sensor itself directly (no more poking).
 
     :param http_conn_id: The connection to run the sensor against
     :type http_conn_id: string