Missing documentation for NATS Server MQTT Parameters

[ColinSullivan1]

It'd be good to have documentation for the following NATS Server MQTT parameters:

• stream_replicas
• consumer_memory_storage
• consumer_inactive_threshold

Note, consumer_replicas is left out because that doesn't seem to be supported at the moment.

Specifically, it'd be valuable to users to understand when to set these (usually when scaling up QoS 1 clients) and provide suggested values. May even want to add a FAQ about that or call it out in the MQTT doc for users who run into scaling issues as it's difficult to diagnose w/o knowing to look into JetStream.

With respect to client scaling, I'd suggest something like:

mqtt {
  stream_replicas=1
  consumer_memory_storage=true
  consumer_inactive_threshold=20s
}