Update google_sql_user documentation for IAM Group authentication for Postgres

[pratikgarg10]

Release Note Template for Downstream PRs (will be copied)

[github-actions]

Hello! I am a robot. Tests will require approval from a repository maintainer to run.

@hao-nan-li, a repository maintainer, has been assigned to review your changes. If you have not received review feedback within 2 business days, please leave a comment on this PR asking them to take a look.

You can help make sure that review is quick by doing a self-review and by running impacted tests locally.

[modular-magician]

Hi there, I'm the Modular magician. I've detected the following information about your changes:

Diff report

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

google provider: Diff ( 1 file changed, 4 insertions(+), 3 deletions(-))
google-beta provider: Diff ( 1 file changed, 4 insertions(+), 3 deletions(-))

[hao-nan-li]

Thanks for the update, release note should be none for doc changes.