docs: update memory and threading API docs

docs/api/memory/pool-allocator/get-total-freed.md

@@ -0,0 +1,33 @@
+# PoolAllocator::GetTotalFreed
+
+```cpp
+size_t GetTotalFreed() const override;
+```
+
+返回当前空闲的内存总量（字节）。计算公式：`m_freeBlocks * m_blockSize`。
+
+**参数：** 无
+
+**返回：** 当前空闲块的字节数
+
+**复杂度：** O(1)
+
+**示例：**
+
+```cpp
+#include <XCEngine/Memory/PoolAllocator.h>
+
+PoolAllocator pool(sizeof(int), 100);
+
+size_t freed = pool.GetTotalFreed(); // 100 * sizeof(int)
+
+void* block = pool.Allocate();
+freed = pool.GetTotalFreed(); // 99 * sizeof(int)
+
+pool.Free(block);
+freed = pool.GetTotalFreed(); // 100 * sizeof(int)
+```
+
+## 相关文档
+
+- [PoolAllocator 总览](pool-allocator.md) - 返回类总览