Update documentation regarding arguments of ValidationError (#8807)

The documentation used to state that the `detail` argument was
mandatory, but in fact it currently is not.

Author: totycro

docs/api-guide/exceptions.md

@@ -217,11 +217,10 @@ By default this exception results in a response with the HTTP status code "429 T
 
 ## ValidationError
 
-**Signature:** `ValidationError(detail, code=None)`
+**Signature:** `ValidationError(detail=None, code=None)`
 
 The `ValidationError` exception is slightly different from the other `APIException` classes:
 
-* The `detail` argument is mandatory, not optional.
 * The `detail` argument may be a list or dictionary of error details, and may also be a nested data structure. By using a dictionary, you can specify field-level errors while performing object-level validation in the `validate()` method of a serializer. For example. `raise serializers.ValidationError({'name': 'Please enter a valid name.'})`
 * By convention you should import the serializers module and use a fully qualified `ValidationError` style, in order to differentiate it from Django's built-in validation error. For example. `raise serializers.ValidationError('This field must be an integer value.')`