Fix math documentation discrepancies

- Vector4.md: Add index mapping (0=x, 1=y, 2=z, 3=w) to operator[] description
- Transform.md: Add proper '运算符' subsection header for operator* documentation

Found and fixed missing operator documentation:
- Vector4: operator[] index mapping was undocumented
- Transform: operator* was listed without proper section header

docs/api/math/transform/transform.md

@@ -28,6 +28,11 @@ struct Transform {
 |------|--------|------|
 | [ToMatrix()](tomatrix.md) | `Matrix4` | 转换为 4x4 变换矩阵 |
 | [Inverse()](inverse.md) | `Transform` | 逆变换 |
+
+### 运算符
+
+| 运算符 | 返回值 | 描述 |
+|--------|--------|------|
 | `operator*(Transform, Transform)` | `Transform` | 组合变换 |
 
 ### 空间变换

docs/api/math/vector4/vector4.md

@@ -45,7 +45,7 @@ struct Vector4 {
 ## 运算符
 
 - 算术: `+`, `-`, `*` (scalar)
-- 下标: `operator[]`
+- 下标: `operator[]` (0=x, 1=y, 2=z, 3=w)
 - 比较: `==`, `!=`
 
 ## 相关文档