Summary:
**Motivation**:
The `CameraRoll.getPhotos()` documentation is lacking, I've simply exposed the shapes of the params object and returned object, as defined and documented in `getPhotosParamChecker` and `getPhotosReturnChecker`.
**Test plan**:
Built the website and checked the documentation.
Closes https://github.com/facebook/react-native/pull/12524
Differential Revision: D4608419
Pulled By: hramos
fbshipit-source-id: 887d32b32312a492e57bf65fca503891de0bd661
4b26ebd063