[docs] [R-package] improve R-package docs on testing (#3724)
[docs] [R-package] improve R-package docs on testing (#3724)


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