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/spinlock/spinlock.md

@@ -22,7 +22,7 @@
 
 ## STL 兼容方法
 
-支持 `lock()`, `unlock()`, `try_lock()` 以兼容 STL 的 lockable 概念。
+支持 `lock()`, `unlock()`, `try_lock()` 以兼容 STL 的 lockable 概念。**注意**：这些方法为非 `const` 成员函数。
 
 ## 使用示例