Update firebase_database_instance docs and samples for region

[modular-magician]

Design Doc: go/rtdb-terraform-modules
Tracking Bug: b/244215914 RTDB Terraform modules

RTDB is only available in 3 regions today: us-central1, europe-west1, asia-southeast1.
https://firebase.google.com/docs/projects/locations#rtdb-locations

The default region (first region launched) is us-central1. The docs page shows us-west1, which RTDB has no support in.

This PR updates docs and samples only. No code changes.

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Updates description for firebase_database_instance.region

Derived from GoogleCloudPlatform/magic-modules#7106