Update WorkerThreadPool doc to mention every task should be waited so that any allocated resources can be cleaned up.

[jsjtxietian]

Fixes #84888

[j-zeppenfeld]

This is certainly the easy way forward, but I wouldn't exactly call it a fix.

The documentation for WorkerThreadPool explicitly states:

This can be used for simple multithreading without having to create Threads.

Having to track task ids and make sure to call wait_for_task_completion without blocking the main thread is not "simple".

[MewPurPur]

I wrote the docs without much knowledge on the subject matter, only some testing. I wouldn't take them for gospel, moreover, this seems like a technical limitation. And it's still simpler than multithreading with Thread.

[kleonc]

Seems this should also resolve #84899 (it's kinda a duplicate of #84888).

While at it, it's probably a good idea to also add some notes to is_task_completed/is_group_task_completed that they return a meaningful value only between adding the task and awaiting its completion / that they error in case of checking completion status of a task already awaited completion.

var id = WorkerThreadPool.add_task(some_task)
...
WorkerThreadPool.is_task_completed(id) # false/true
...
WorkerThreadPool.wait_for_task_completion(id)
WorkerThreadPool.is_task_completed(id) # false, and "Invalid Task ID" error

godot/core/object/worker_thread_pool.cpp

Lines 293 to 305 in d5217b6

	bool WorkerThreadPool::is_task_completed(TaskID p_task_id) const {
	task_mutex.lock();
	const Task *const *taskp = tasks.getptr(p_task_id);
	if (!taskp) {
	task_mutex.unlock();
	ERR_FAIL_V_MSG(false, "Invalid Task ID"); // Invalid task
	}
	bool completed = (*taskp)->completed;
	task_mutex.unlock();
	return completed;
	}

godot/core/object/worker_thread_pool.cpp

Lines 486 to 496 in d5217b6

	bool WorkerThreadPool::is_group_task_completed(GroupID p_group) const {
	task_mutex.lock();
	const Group *const *groupp = groups.getptr(p_group);
	if (!groupp) {
	task_mutex.unlock();
	ERR_FAIL_V_MSG(false, "Invalid Group ID");
	}
	bool completed = (*groupp)->completed.is_set();
	task_mutex.unlock();
	return completed;
	}

[RandomShaper]

A fire-and-forget API could be added in the future, provided tracking task ids is really that painful. In the meantime, this PR adds sensitve clatifications.

[akien-mga]

Thanks!