docs: Fix containers module documentation discrepancies

- Array::SetAllocator: Remove reference to non-existent PoolAllocator class
- HashMap::SetAllocator: Remove reference to non-existent GetDefaultAllocator()
- HashMap::Copy/Move: Fix move constructor complexity (O(m_bucketCount), not O(1))
- HashMap::iterator: Remove C++20 structured bindings example
- String: Add missing links for operator+ and operator==/!=

docs/api/containers/hashmap/iterator.md

@@ -27,16 +27,6 @@ for (auto it = map.begin(); it != map.end(); ++it) {
     std::cout << it->first << " -> " << it->second << std::endl;
 }
 // 输出顺序不确定，取决于哈希桶的内部布局
-
-// 范围 for 循环（需要 C++20 或手动实现 begin/end）
-for (auto& [key, value] : map) {
-    std::cout << key << " -> " << value << std::endl;
-}
-
-// 常量迭代器
-for (auto it = map.begin(); it != map.end(); ++it) {
-    std::cout << it->first << " -> " << it->second << std::endl;
-}
 ```
 
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