Add more details on threadcount setting

[pwonson]

Describe the bug The following information was provided by support:

An example situation where increasing the threadpool might be recommended is when Rundeck has 200 scheduled jobs to trigger simultaneously with a thread count of 10 (the default). At the scheduled time a maximum of 10 will run and sometimes less due to Rundeck using some threads for a temporary background process or the job itself using multiple threads. The remaining ~190 jobs are queued. Rundeck executes the waiting jobs as threads are released as jobs complete. Eventually all the queued jobs are trigger, but the start time is delayed.

As you increase the thread count, you must also increase the amount of memory allocated to Rundeck. If the threads count is too high, the simultaneous jobs use too much memory, and can cause Rundeck to run out of memory and go offline. In this case the remaining queued jobs are not executed (). Note that jobs which run locally, are long running, or create a large execution log put a higher load on the host system and must be balanced against the amount of JVM memory allocated to Rundeck, threadcount, and system memory when tuning the system.

Rundeck recommends increasing the threadcount incrementally (50, 100, 200, etc) and testing to ensure that the instance can handle the workload. Remote execution load policy can balance the load across the cluster in Rundeck Enterprise Edition.

Source page https://docs.rundeck.com/docs/administration/maintenance/tuning-rundeck.html#quartz-job-threadcount