docs: clarify google_sql_database_instance edition default (PG16 → ENTERPRISE_PLUS) and tier/edition compatibility #30361

Job	Run time
docs-examples / generate-repository	36s
terraform-google-conversion / generate-repository	39s
terraform-provider-google-beta / generate-repository	1m 16s
terraform-provider-google / generate-repository	1m 18s
terraform-provider-google-beta / unit-test-terraform-provider-google-beta	7m 31s
terraform-google-conversion / compile-and-unit-tests	6m 41s
terraform-provider-google / unit-test-terraform-provider-google	7m 27s
	25m 28s

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