Add dart to JSON schema. (#5218)

* Add dart to JSON schema.

* Use spaces instead of tab.

doc/tutorials/saving_model.rst

@@ -201,6 +201,8 @@ Another important feature of JSON format is a documented `Schema
 XGBoost.  Here is the initial draft of JSON schema for the output model (not
 serialization, which will not be stable as noted above).  It's subject to change due to
 the beta status.  For an example of parsing XGBoost tree model, see ``/demo/json-model``.
+Please notice the "weight_drop" field used in "dart" booster.  XGBoost does not scale tree
+leaf directly, instead it saves the weights as a separated array.
 
 .. include:: ../model.schema
    :code: json