[python][docs] Refer to string type as `str` in docstrings (#4565)
[python][docs] Refer to string type as `str` in docstrings (#4565)


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