fixes #23464: update grid migration docs to mention order replaced pu…

…sh/pull

docs/4.0/migration.md

@@ -139,7 +139,8 @@ New to Bootstrap 4 is the [Reboot]({{ site.baseurl }}/docs/{{ site.docs_version
 ### Grid system
 
 - Added a new `576px` grid breakpoint as `sm`, meaning there are now five total tiers (`xs`, `sm`, `md`, `lg`, and `xl`).
-- Renamed the responsive grid modifier classes from `.col-{breakpoint}-{modifier}-{size}` to `.{modifier}-{breakpoint}-{size}` for simpler grid classes. For example, instead of `.col-md-3.col-md-push-9` it's `.col-md-3.push-md-9`.
+- Renamed the responsive grid modifier classes from `.col-{breakpoint}-{modifier}-{size}` to `.{modifier}-{breakpoint}-{size}` for simpler grid classes.
+- Dropped push and pull modifier classes for the new flexbox-powered `order` classes. For example, instead of `.col-8.push-4` and `.col-4.pull-8`, you'd use `.col-8.order-2` and `.col-4.order-1`.
 - Added flexbox utility classes for grid system and components.
 
 ### List groups