docs: clarify google_sql_database_instance edition default (PG16 → ENTERPRISE_PLUS) and tier/edition compatibility

[pete-csyn]

Summary

Docs-only clarification for google_sql_database_instance. The edition argument doc currently reads only:

edition - (Optional) The edition of the instance, can be ENTERPRISE or ENTERPRISE_PLUS.

It does not mention that, since the client-side ENTERPRISE default was removed (#19977), omitting edition lets the Cloud SQL API choose the default — and for POSTGRES_16+ that default is ENTERPRISE_PLUS. ENTERPRISE_PLUS accepts only db-perf-optimized-N-* (N2/C4A) tiers, so a config that sets a shared-core or custom tier (db-g1-small, db-f1-micro, db-custom-*) without edition now fails at apply with:

Error 400: Invalid request: Invalid Tier (db-g1-small) for (ENTERPRISE_PLUS) Edition.
Use a predefined Tier like db-perf-optimized-N-* instead.

The existing tier doc actively points PostgreSQL users at exactly those shared-core/custom tiers, with no cross-reference to the edition interaction — so the docs steer users into the failure. This was reported in hashicorp/terraform-provider-google#20498 (closed without a docs fix), where the community-confirmed remediation is edition = "ENTERPRISE".

This PR adds that context to both the edition and tier argument descriptions. No code/schema change.

Release Note

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