Explain use case for `display_value` in docs

docs/api-guide/relations.md

@@ -539,7 +539,7 @@ to ``True``.
 
 ## The `display_value` method
 
-The `__str__` (`__unicode__` on Python 2) method of the model will be called to generate string representations of the objects used to populate the `choices` property. To provide customized representations, override `display_value` of a `RelatedField` subclass. This method will receive a model object, and should return a string suitable for representing it. For example:
+The `__str__` (`__unicode__` on Python 2) method of the model will be called to generate string representations of the objects used to populate the `choices` property. These choices are used to populate select HTML inputs in the browsable API. To provide customized representations for such inputs, override `display_value` of a `RelatedField` subclass. This method will receive a model object, and should return a string suitable for representing it. For example:
 
     class TrackPrimaryKeyRelatedField(serializers.PrimaryKeyRelatedField):
         def display_value(self, instance):