Document new max_inflight behaviour for NATS queue worker (async reference page)

[alexellis]

Proposed change

Document new max_inflight behaviour for NATS queue worker (async reference page)

Context

After openfaas/nats-queue-worker#95 was merged, we need to update the documentation to explain that A) concurrency can now be controlled via this env-var and B) HA can be controlled by replicas.

We should also make the flag available in the helm chart if it is not already, and in the CLI installer for OpenFaaS (arkade)

[matthiashanel]

@alexellis will do. the pr came with a suggested doc change. were. you ok with that?

[matthiashanel]

Basically adding:
Alternatively you can increase the parallelism by setting "max_inflight" to a value greater than one. This will cause the queue-worker to concurrently receive up to max_inflight many messages and simultaneously invoke their corresponding functions. Should you wish to restrict concurrent invocation for certain functions, please make use of the queue feature and separate these from functions where you want more concurrency. When you scale up a queue worker, please be aware that you will get n * max_inflight parallel tasks.