docs(api): document canMangle (#6257)

Author: chenxsan

src/content/api/module-variables.mdx

@@ -264,6 +264,8 @@ In modules, `__webpack_exports_info__` is available to allow exports introspecti
 
 - Accessing the info from nested exports is possible: i. e. `__webpack_exports_info__.<exportName>.<exportName>.<exportName>.used`
 
+- Check whether exports can be mangled with `__webpack_exports_info__.<name>.canMangle`
+
 ## \_\_webpack_is_included\_\_ (webpack-specific)
 
 <Badge text="5.16.0+" />