Better documentation for router getScreenOptions

[voxspox]

The documentation of getScreenOptions needs some clarification:

• the headline is getScreenOptions(navigation, screenProps) but the text explains 3 parameters. What is about navigationOptions?
• The text says "retrieve the navigation options for a screen". Why the function is named screen options while it returns navigation options?
• The example code uses addNavigationHelpers which does not exits anymore?
• For me is not clear where I can use this function, p.ex. in screen, in my custom router, etc. Please improve the example to make this more clear.

[brentvatne]

unless you have some very specific and probably unusual use case you shouldn't need to used this, createNavigator will use it for computing the screen descriptors. i'd suggest reading code to get some more insight into how this works if you're curious, and if you have time to improve the documentation around it that would be very welcome!

some good places to learn:

https://github.com/react-navigation/react-navigation-core/blob/10b3544c78700bed30e2ff31d174dec0f899ec27/src/routers/SwitchRouter.js#L341-L344
https://github.com/react-navigation/react-navigation-core/blob/10b3544c78700bed30e2ff31d174dec0f899ec27/src/routers/createConfigGetter.js
https://github.com/react-navigation/react-navigation-core/blob/10b3544c78700bed30e2ff31d174dec0f899ec27/src/navigators/createNavigator.js#L32-L57