Update iot-hub-devguide-query-language.md

Author: ash2017

articles/iot-hub/iot-hub-devguide-query-language.md

@@ -476,7 +476,7 @@ In routes conditions, the following type checking and casting functions are supp
 | AS_NUMBER | Converts the input string to a number. `noop` if input is a number; `Undefined` if string does not represent a number.|
 | IS_ARRAY | Returns a Boolean value indicating if the type of the specified expression is an array. |
 | IS_BOOL | Returns a Boolean value indicating if the type of the specified expression is a Boolean. |
-| IS_DEFINED | Returns a Boolean indicating if the property has been assigned a value. This is supported only when the value is a primitive. DateTime, object types and arrays are not supported. |
+| IS_DEFINED | Returns a Boolean indicating if the property has been assigned a value. This is supported only when the value is a primitive type. Primitive types include string, Boolean, numeric, or `null`. DateTime, object types and arrays are not supported. |
 | IS_NULL | Returns a Boolean value indicating if the type of the specified expression is null. |
 | IS_NUMBER | Returns a Boolean value indicating if the type of the specified expression is a number. |
 | IS_OBJECT | Returns a Boolean value indicating if the type of the specified expression is a JSON object. |