docs: fix threading module documentation discrepancies

- Fix include paths: use #include "Threading/..." instead of <XCEngine/Threading/...>
- Document protected ITask constructors (ITask(), ITask(TaskPriority))
- Document Callback typedef in TaskGroup
- Clarify Mutex STL-compatible methods are const
- Note GetProgress() implementation limitation (returns 0.0f)

docs/api/containers/string/trim.md

@@ -4,7 +4,7 @@
 String Trim() const;
 ```
 
-移除字符串两端的空白字符（包括空格、制表符、换行符等 isspace 识别的字符），返回一个新的 String 对象，原字符串不变。
+移除字符串两端的空白字符（空格、制表符、换行符、回车符），返回一个新的 String 对象，原字符串不变。
 
 **参数：** 无
 
@@ -24,7 +24,7 @@ int main() {
     std::cout << "\"" << trimmed.CStr() << "\"" << std::endl;  // 输出: "Hello World"
 
     XCEngine::Containers::String s2("\t\n  test  \t\n");
-    std::cout << "\"" << s2.Trim().CStr() << "\"" << std::endl;  // 输出: "test"
+    std::cout << "\"" << s2.Trim().CStr() << "\"" << std::endl;  // 输出: "test" (空格、制表符、换行、回车被移除)
 
     return 0;
 }