Backport of docs: improve write-only attribute example into v1.11

[github-actions]

Backport

This PR is auto-generated from #36605 to be assessed for backporting due to the inclusion of the label 1.11-backport.

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The person who merged in the original PR is:
@bschaatsbergen
This person should manually cherry-pick the original PR into a new backport PR,
and close this one when the manual backport PR is merged in.

merge conflict error: unable to process merge commit: "8e0f8bb468ad1d0b322ce79eaff4ec4e7f67878d", automatic backport requires rebase workflow

The below text is copied from the body of the original PR.

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This PR makes a few improvements to the write-only arguments example configuration in the documentation. Along with fixing a couple of incorrect arguments—like the ephemeral resource name, the invalid db.t5.micro RDS instance type, and the use of admin as a reserved master username in RDS—this update also improves the workflow to better match what end users typically expect.

Without these changes, the example would not work and the password is essentially lost when it's generated and written to the database without a way to capture it for future use. The updated workflow now writes the ephemeral random_password to AWS Secret Manager using a write-only attribute, then retrieves it through an ephemeral aws_secretsmanager_secret_version resource (which is deferred to the apply stage due to a computed argument) and finally it securely passes the password into the write-only attribute of the aws_db_instance managed resource.

By making these changes, we're able to both capture the random password and securely pass it to the AWS RDS.

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[bschaatsbergen]

Waiting before we manually back port this, to include #36621