Update form validation migration docs

docs/4.0/migration.md

@@ -120,8 +120,7 @@ New to Bootstrap 4 is the [Reboot]({{ site.baseurl }}/docs/{{ site.docs_version
   - Added new `.form-control-label` class to vertically center labels with `.form-control`s.
   - Added new `.form-row` for compact form layouts with the grid classes (swap your `.row` for a `.form-row` and go).
 - Added custom forms support (for checkboxes, radios, selects, and file inputs).
-- Added HTML5 form validation support via CSS's `:invalid` and `:valid` pseudo-classes.
-- Renamed `.has-error` to `.has-danger`.
+- Replaced `.has-error`, `.has-warning`, and `.has-success` classes with HTML5 form validation via CSS's `:invalid` and `:valid` pseudo-classes.
 - Renamed `.form-control-static` to `.form-control-plaintext`.
 
 ### Buttons