Calling XGBModel.fit() should clear the Booster by default (#6562)
Calling XGBModel.fit() should clear the Booster by default (#6562)

* Calling XGBModel.fit() should clear the Booster by default

* Document the behavior of fit()

* Allow sklearn object to be passed in directly via xgb_model argument

* Fix lint
dmlc/xgboost,dmlc/xgboost,dmlc/xgboost,dmlc/xgboost,dmlc/xgboost,dmlc/xgboost