Document new Search component in change logs.

docs/en/04_Changelogs/4.3.0.md

@@ -5,6 +5,7 @@
  - `DataList::column()` now returns all values and not just "distinct" values from a column as per the API docs
  - `DataList`, `ArrayList` and `UnsavedRalationList` all have `columnUnique()` method for fetching distinct column values
  - Take care with `stageChildren()` overrides. `Hierarchy::numChildren() ` results will only make use of `stageChildren()` customisations that are applied to the base class and don't include record-specific behaviour.
+ - New react-based search UI for the CMS, Asset-Admin, GridFields and ModelAdmins.
 
 ## Upgrading {#upgrading}
 
@@ -12,3 +13,30 @@
 
 Prior to this release `DataList` would erroneously return a distinct list of values from a column on an object.
 If this behaviour is still required, please use `columnUnique()` instead.
+
+### Using legacy GridField search API
+
+GridFields now default to using the new search UI which uses [SilverStripe's FormSchema API](https://api.silverstripe.org/master/SilverStripe/Forms/Schema/FormSchema.html).
+
+If you would rather continue using the old search API, you can remove the default `GridFieldFilterHeader` from your GridField configuration and replace with one whose _legacy_ flag has been enabled. 
+
+To enable legacy mod on a `GridFieldFilterHeader`, just pass `true` to the first argument of its constructor.
+
+```php
+public function getCMSFields()
+{
+    $fields = parent::getCMSFields();
+
+    // Configure grid field to use legacy search API
+    $legacySearchApiFlag = true;
+    $config = new GridFieldConfig_RecordEditor();
+    $config->removeComponentsByType(GridFieldFilterHeader::class);
+    $config->addComponent(new GridFieldFilterHeader($legacySearchApiFlag));
+    
+    $grid = new GridField('Companies', 'Companies', new DataList(Company::class), $config);
+    $fields->addFieldToTab('Root.Compagnu', $grid);
+    
+    return $fields;
+}
+
+```