Update documentation for google_sql_user IAM auth.

[fotinakis]

Update documentation for google_sql_user to include example usage for CLOUD_IAM_SERVICE_ACCOUNT, including the correct trimsuffix usage required for GCP Postgres.

Fixes hashicorp/terraform-provider-google#14019

(removed non-applicable acknowledgements)

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[modular-magician]

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[fotinakis]

Signed CLA

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Diff report

Your PR generated some diffs in downstreams - here they are.

Terraform GA: Diff ( 1 file changed, 11 insertions(+), 3 deletions(-))
Terraform Beta: Diff ( 1 file changed, 11 insertions(+), 3 deletions(-))
TF Conversion: Diff ( 2 files changed, 3 insertions(+), 3 deletions(-))