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/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