Update google_sql_database_instance documentation

[megan07]

settings.backup_configuration.binary_log_enabled may be true even if settings.backup_configuration.enabled is false.

This behaviour as changed in #4907

Upstreams hashicorp/terraform-provider-google#10951

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[modular-magician]

Hi! I'm the modular magician. Your PR generated some diffs in downstreams - here they are.

Diff report:

Terraform GA: Diff ( 1 file changed, 1 insertion(+), 2 deletions(-))
Terraform Beta: Diff ( 1 file changed, 1 insertion(+), 2 deletions(-))