Add note about Postgres requiring user passwords

[rileykarson]

Upstreams hashicorp/terraform-provider-google#5569

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[rileykarson]

This is a third_party dir where the CLA does not apply.

[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(-))