Document allow_empty argument (#4117)

ListSerializer fields or serializers that are passed many=True may also take an allow_empty=False argument to disallow empty lists as valid input.

Information about this was part of the 3.2 release announcement, but had not been part of the API docs until now.

docs/api-guide/serializers.md

@@ -731,9 +731,17 @@ The `ListSerializer` class provides the behavior for serializing and validating
 
 When a serializer is instantiated and `many=True` is passed, a `ListSerializer` instance will be created. The serializer class then becomes a child of the parent `ListSerializer`
 
+The following argument can also be passed to a `ListSerializer` field or a serializer that is passed `many=True`:
+
+### `allow_empty`
+
+This is `True` by default, but can be set to `False` if you want to disallow empty lists as valid input.
+
+### Customizing `ListSerializer` behavior
+
 There *are* a few use cases when you might want to customize the `ListSerializer` behavior. For example:
 
-* You want to provide particular validation of the lists, such as always ensuring that there is at least one element in a list.
+* You want to provide particular validation of the lists, such as checking that one element does not conflict with another element in a list.
 * You want to customize the create or update behavior of multiple objects.
 
 For these cases you can modify the class that is used when `many=True` is passed, by using the `list_serializer_class` option on the serializer `Meta` class.