IAM policies on Secret Manager on single Elastic CI Stack note

[james2791]

This change:

• Adds a note underneath the main page within the docs/pages/agent/v3/aws/secrets_manager.md page (Storing agent tokens) to clarify the need for only one IAM role to be set in a corresponding secret's policy.
• Came out from community forum discussion and testing (Autoscaling Lambda/components not made in the single case)

[render]

Your Render PR Server URL is https://bk-docs-pr-2161.onrender.com.

Follow its progress at https://dashboard.render.com/web/srv-chqmav8rddleuf4glg9g.

[mbelton-buildkite]

I think this is a little simpler/clearer:

Suggested change

	>📘 Single instance Elastic CI Stacks
	> If you have set `minSize` and `maxSize` parameters equal to 1 in your [Elastic CI Stack parameters](/docs/agent/v3/elastic-ci-aws/parameters), you will only need to set the `IAMRole` to your Secret Manager secret's resource policy as above. The `AutoscalingLambdaExecutionRole` IAM role and corresponding autoscaling resources are not created in this Elastic CI Stack setup.
	>📘 Single instance stacks
	> If you have set `minSize` and `maxSize` [parameters](/docs/agent/v3/elastic-ci-aws/parameters) equal to `1`, don't include the `AutoscalingLambdaExecutionRole` IAM role in the resource policy. Only the `IAMRole` is required since this setup doesn't need autoscaling resources.

But it's a little less precise, so happy to go with your version if you think it works better. Let me know what you think.