Update to more clearly define options with Nest (#1513)

Based on discussion in this thread.  https://community.home-assistant.io/t/nest-protect-configuration/1327/11

source/_components/sensor.nest.markdown

@@ -38,19 +38,25 @@ sensor:
 Configuration variables:
 
 - **monitored_conditions** array (*Required*): States to monitor.
-  - 'temperature'
+
-  - 'target'
+The following conditions can be monitored with a Nest Thermostat or Protect.
-  - 'humidity'
+  - 'battery_level'
-  - 'operation_mode'
+  
-  - 'last_ip'
+The following conditions can be monitored with a Nest Thermostat only.
-  - 'local_ip'
+  - 'temperature'
-  - 'last_connection'
+  - 'target' 
-  - 'battery_level'
+  - 'humidity'  
-  - 'weather_condition'
+  - 'operation_mode'  
-  - 'weather_temperature'
+  - 'last_ip'  
-  - 'weather_humidity'
+  - 'local_ip' 
-  - 'wind_speed'
+  - 'last_connection'  
-  - 'wind_direction'
+  - 'weather_condition'  
-  - 'co_status' # Nest Protect only
+  - 'weather_temperature' 
-  - 'smoke_status' # Nest Protect only
+  - 'weather_humidity' 
+  - 'wind_speed'  
+  - 'wind_direction'  
+  
+The following conditions can be monitored with a Nest Protect only. 
+  - 'co_status'
+  - 'smoke_status'