docs: 更新 containers 和 threading 模块文档

- containers: 更新 string 类的多个方法文档
- threading: 更新 mutex 和 task-group 方法文档

docs/api/threading/mutex/unlock-const.md

@@ -19,7 +19,7 @@ void unlock() const;
 ```cpp
 #include "XCEngine/Threading/Mutex.h"
 
-XCEngine::XCEngine::Threading::Mutex mtx;
+XCEngine::Threading::Mutex mtx;
 std::vector<int> data;
 
 void SafePush(int value) {