docs: update memory and threading API docs

docs/api/memory/pool-allocator/get-allocation-count.md

@@ -0,0 +1,40 @@
+# PoolAllocator::GetAllocationCount
+
+```cpp
+size_t GetAllocationCount() const override;
+```
+
+返回当前已分配块的数量。计算公式：`m_totalBlocks - m_freeBlocks`。
+
+**参数：** 无
+
+**返回：** 当前已分配块的数量
+
+**复杂度：** O(1)
+
+**示例：**
+
+```cpp
+#include <XCEngine/Memory/PoolAllocator.h>
+
+PoolAllocator pool(sizeof(int), 100);
+
+size_t count = pool.GetAllocationCount(); // 0
+
+void* blocks[10];
+for (int i = 0; i < 10; ++i) {
+    blocks[i] = pool.Allocate();
+}
+
+count = pool.GetAllocationCount(); // 10
+
+for (int i = 0; i < 5; ++i) {
+    pool.Free(blocks[i]);
+}
+
+count = pool.GetAllocationCount(); // 5
+```
+
+## 相关文档
+
+- [PoolAllocator 总览](pool-allocator.md) - 返回类总览