LightGBM

[docs][examples] Clarify relationship between `xentropy` and `binary` (#1382)

* Note the relationship between `binary` and `xentropy` in the docs and provide an example that compares them

* Pass pylint and fix docs formatting. "pylint: disable = no-name-in-module" because pylint does not believe scipy.special contains expit

* pass pycodestyle

* pass pycodestyle with correct flags; document example in readme