link rational, in case we don't rollback

docs/src/pages/guides/migration-v4/migration-v4.md

@@ -394,6 +394,7 @@ const classes = makeStyles(theme => ({
   ```
 
   [This codemod](https://github.com/mui-org/material-ui/tree/HEAD/packages/material-ui-codemod#box-sx-prop) will automatically update your code to the new syntax.
+  You can [read this section](/system/basics/#api-tradeoff) for the why behind the change of API.
 
 - The `borderRadius` system prop value transformation has been changed. If it receives a number, it multiplies this value with the `theme.shape.borderRadius` value. Use a string to provide an explicit value, in `px`.