Summary:
Update documentation for Switch as it's not a background color that is displayed when the switch off, but juste the border (the background remains transparent)
Closes https://github.com/facebook/react-native/pull/9707
Differential Revision: D3819887
Pulled By: mkonicek
fbshipit-source-id: 218fd20deaded5260cc51e815b559f3cbbb3f752