[DOCS] Discuss 502/504 error when stopping EC2 instance during Session Stickyness Demo

[james-leatherman]

During [SHAREDALL] [DEMO] Seeing Session Stickiness in Action, during Stage 1G we are directed to stop the EC2 instance that the client becomes sticky with. This stop action leads immediately to a 502 Bad Gateway error for about 30-60 sec., then a 504 Gateway Time-out error after that for about the same time. Then the LB correctly routes a running instance.

For students unfamiliar with how the ALB does health checks, this will be distracting/disorienting and will seem like a configuration error. Add a note in the lab instructions that describe this behavior so that students know that they have successfully completed the lab despite the behavior.