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)

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