WorkerThreadPool docs do not specify how Thread count is set

**Your Godot version:**
4.2

**Issue description:**
The docs for WorkerThreadPool say the following:
> A singleton that allocates *some* Threads on startup
> The WorkerThreadPool singleton allocates *a set* of Threads (called worker threads) on project startup

This is vague, and does not explain how WorkerThreadPool decides how many Threads to create (nor does WorkerThreadPool provide any way to either configure this or check the current thread count).

This creates a lot of confusion trying to use WorkerThreadPool efficiently alongside other manually created Threads.

**URL to the documentation page (if already existing):**
https://docs.godotengine.org/en/stable/classes/class_workerthreadpool.html