[docs][python][scikit-learn] added note for LGBMRanker (#4243)
[docs][python][scikit-learn] added note for LGBMRanker (#4243)


henry0312/LightGBM,henry0312/LightGBM,microsoft/LightGBM,microsoft/LightGBM,microsoft/LightGBM,henry0312/LightGBM,microsoft/LightGBM,henry0312/LightGBM,microsoft/LightGBM,henry0312/LightGBM