react-native-firebase

docs: update database usage with switching database URL (#3431)

* docs: update database usage with switching database URL

I struggled a while with finding out how to switch the database URL for the [DEFAULT] firebase instance during runtime.
With version 5 it was very easy with simply passing the databaseURL to the database module.

The docs did not help unfortunately, so I tried a lot of ways which are documented in this ticket:
https://github.com/invertase/react-native-firebase/issues/3049

This PR shall help other people with the same situation too.

Furthermore, I hope to get valid feedback if the proposed implementation documented in this PR is actually correct.

* refactor: move section above firebase.json

* refactor: change section title

* refactor: remove title, fix typo

* refactor: fix typo

* refactor: feedback from review