[docs] Completing the useFocusEffect doc example

[kimchouard]

Why

To be as exhaustive as the react-navigation equivalent

How

Added the return () => {} function to the useFocusEffect example, to show how to trigger code when view gets out of focus.

Test Plan

N/A

Checklist

• Documentation is up to date to reflect these changes (eg: https://docs.expo.dev and README.md).
• Conforms with the Documentation Writing Style Guide
• This diff will work correctly for npx expo prebuild & EAS Build (eg: updated a module plugin).

[amandeepmittal]

Thanks for updating this example @kimchouard!

Congratulations on your first contribution 🎉

I'm just going to make a small change to the log message to make it more meaningful and then merge this PR.