docs: fix threading module documentation discrepancies

- Fix SpinLock docs: clarify lock()/unlock()/try_lock() are non-const
  (matching source implementation in SpinLock.h)
- Add missing LambdaTask::Execute() method to inherited methods table
- Update TaskGroup::Wait() docs: clarify m_pendingCount never decrements
  causing indefinite block even after all tasks complete (not just
  unexecuted tasks)
- Update TaskGroup::IsComplete() docs: document same m_pendingCount
  issue causing incorrect return values

docs/api/threading/task-group/iscomplete.md

@@ -12,6 +12,8 @@ bool IsComplete() const
 
 **复杂度：** O(1)
 
+**注意：** 由于实现缺陷，`m_pendingCount` 计数器在任务完成后不会递减，因此此方法在所有任务执行完毕后仍会错误地返回 false。
+
 **示例：**
 
 ```cpp