[expo-docs] Document Segment.screen|screenWithProperties()
[expo-docs] Document Segment.screen|screenWithProperties()

- for https://github.com/expo/expo/pull/358

Closes expo/expo-docs#101

exponentjs/exponent,exponent/exponent,exponentjs/exponent,exponent/exponent,exponent/exponent,exponent/exponent,exponentjs/exponent,exponent/exponent,exponentjs/exponent,exponent/exponent,exponent/exponent,exponentjs/exponent,exponentjs/exponent,exponentjs/exponent,exponent/exponent,exponentjs/exponent,exponent/exponent,exponentjs/exponent