Remove the detail=None from APIException signature

The documentation not match with the implementation. The APIException doesn't have detail parameter in the constructor class, actually doesn't have constructor method at all.

docs/api-guide/exceptions.md

@@ -82,7 +82,7 @@ Note that the exception handler will only be called for responses generated by r
 
 ## APIException
 
-**Signature:** `APIException(detail=None)`
+**Signature:** `APIException()`
 
 The **base class** for all exceptions raised inside REST framework.