Clarify single administrative user limitation for RabbitMQ brokers #41374

[laryssasre]

Fixes #41374.

This PR updates the documentation for the aws_mq_broker resource to clarify that when using Amazon MQ with engine_type = "RabbitMQ", only one administrative user can be created during broker provisioning.

Specifically:

• Adds a new detailed note explaining that:
  • Only one admin user is allowed at creation time for RabbitMQ brokers.
  • Additional users must be created after provisioning via the RabbitMQ Management API or Amazon MQ console.
  • Terraform cannot manage users post-provisioning; modifying the user block triggers full broker recreation (ForceNew behavior).
  • Amazon MQ does not expose RabbitMQ users via APIs, making drift detection impossible.
• Updates the initial resource description to clarify that user management happens only at creation time.
• Removes the redundant NOTE at the end of the user block, now covered by the more detailed note earlier in the documentation.

References:

• AWS CloudFormation documentation for AWS::AmazonMQ::Broker.User

[github-actions]

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