The Concurrency section of the PowerShell developer guide is outdated

The [Concurrency](https://learn.microsoft.com/en-us/azure/azure-functions/functions-reference-powershell?tabs=portal#concurrency) section needs an overhaul. Specifically:

- It claims that by default the Functions PowerShell runtime can only process one invocation of a function at a time. This was true in Functions 3.x, and this is not true in Functions 4.x.
- It mentions increasing `FUNCTIONS_WORKER_PROCESS_COUNT` as the first option to increase concurrency. Except for rare corner cases, creating more than one PowerShell worker process per instance introduces additional overhead without providing enough benefits. We should deemphasize this option (perhaps move it to the end) and make the caveats more prominent.
- The default value of `PSWorkerInProcConcurrencyUpperBound` in Functions 4.x is 1000 (as opposed to 1 in Functions 3.x). So, in Functions 4.x it rarely needs to be increased.
- This section should probably start with a claim that the default values are good enough in most cases.