[DOCS] Improve worker_threads documentation

Description
I'm struggling to get a feel for how to tune the worker_threads value for a multi-syndic/multi-master setup. I'm seeing issues with the syndic processes being able to call fire_master and also not being able to publish results to the master, however our monitoring does not indicate any excess load or CPU utilization on any of the masters. Currently this is set to 4 times the number of cores of the master host. Trying to figure this out be trial and error is quite difficult on a production cluster.

Suggested Fix
Some indication of how this parameter should scale with the number of cores and how to determine whether increasing the number of worker threads is appropriate would be really helpful, the worker_threads parameter isn't even mentioned on your Salt at Scale page.

Type of documentation
Salt documentation

Location or format of documentation
https://docs.saltproject.io/en/latest/ref/configuration/master.html#worker-threads

Start with the worker_threads section of ref/configuration/master.html and compare it with the Salt at Scale page linked in the issue. Investigate the requested multi-syndic/multi-master context and document how the setting relates to host cores and how operators can tell whether more threads are appropriate. Done means the configuration reference and scale guidance address these questions.