Update database config explanation in with-firebase-app example (#20709)
Update database config explanation in with-firebase-app example (#20709)

I made the following changes.

- Addition of procedure for setting private_key
- Addition of procedure for creating a database and setting database_url
flybayer/next.js,JeromeFitz/next.js,azukaru/next.js,azukaru/next.js,zeit/next.js,zeit/next.js,azukaru/next.js,JeromeFitz/next.js,JeromeFitz/next.js,flybayer/next.js,azukaru/next.js,flybayer/next.js,JeromeFitz/next.js,zeit/next.js,flybayer/next.js