docs: Fix memory module documentation discrepancies

- Add missing PoolAllocator class overview with methods table
- Add missing LinearAllocator class overview with methods table
- Add missing ProxyAllocator class overview with methods table
- Fix PoolAllocator::Allocate example code and comments
- Clarify ProxyAllocator::Free totalFreed calculation behavior
- Fix CreateLinearAllocator complexity from O(size) to O(1)
- Add note about Reallocate thread safety in ProxyAllocator

docs/api/memory/pool-allocator/allocate.md

@@ -19,11 +19,12 @@ void* Allocate(size_t size, size_t alignment = 0) override;
 ```cpp
 #include <XCEngine/Memory/PoolAllocator.h>
 
-PoolAllocator pool(sizeof(int) * 100, 50, alignof(int));
+// 创建能容纳 1000 个整数的内存池
+PoolAllocator pool(sizeof(int), 1000, alignof(int));
 
-// 分配（忽略 size 参数）
-void* block1 = pool.Allocate(1);     // 分配 1 字节
-void* block2 = pool.Allocate(10000); // 也分配一块
+// 分配（忽略 size 参数，只分配固定大小的块）
+void* block1 = pool.Allocate(1);     // 分配一块（实际大小为 sizeof(int)）
+void* block2 = pool.Allocate(10000); // 也分配一块（实际大小为 sizeof(int)）
 
 if (block1 && block2) {
     // 使用分配的内存块