docs: Clarify use of public_cluster_addr for load balancing

[Dan-Heath]

Description

From a Slack conversation: https://hashicorp.slack.com/archives/C012RTGJR1V/p1753204358604259

A customer misunderstood the way the public_cluster_addr parameter worked with regard to load balancing. They thought the setting required the address of the controller node and not the cluster. Jeff Mitchell confirmed that the documentation was correct, but that we could better explain how load balancing worked.

This PR updates that parameter description to convey that workers must know the addresses of the controllers to perform load balancing and not just the address of the listener itself. It also provides an example of how someone might use this setting with a load balancer.

View the update in the preview deployment

PCI review checklist

• I have documented a clear reason for, and description of, the change I am making.
• If applicable, I've documented a plan to revert these changes if they require more than reverting the pull request.
• If applicable, I've documented the impact of any changes to security controls.
  Examples of changes to security controls include using new access control methods, adding or removing logging pipelines, etc.