Update description of docs/api-guide/fields.md -> required

fix: Documentation did not point out that default value of `required`
`Field` parameter is `False` depending on the Django's `Model.field`.

Author: Jakub Peczke

docs/api-guide/fields.md

@@ -42,7 +42,7 @@ Set to false if this field is not required to be present during deserialization.
 
 Setting this to `False` also allows the object attribute or dictionary key to be omitted from output when serializing the instance. If the key is not present it will simply not be included in the output representation.
 
-Defaults to `True`.
+Defaults to `True`. If you're using [Model Serializer](https://www.django-rest-framework.org/api-guide/serializers/#modelserializer) default value will be `False` if you have specified `blank=True` or `default` or `null=True` at your field in your `Model`.
 
 ### `default`