Fix R documentation for xgb.train. (#6764)
Fix R documentation for xgb.train. (#6764)


The [general documentation](https://xgboost.readthedocs.io/en/latest/parameter.html#parameters-for-tree-booster) clearly has alpha and lambda under its "Parameters for Tree Booster" heading. Furthermore, the R package clearly uses alpha and lambda when told to use the tree booster. This update adds those two parameters to the documentation for the R package.


Closed issue #6763.
dmlc/xgboost,dmlc/xgboost,dmlc/xgboost,dmlc/xgboost,dmlc/xgboost,dmlc/xgboost