MDL-78288 formslib: Document new filter_shown_headers function in For…

…ms API

docs/apis/subsystems/form/index.md

@@ -300,6 +300,30 @@ public function definition() {
 }
 ```
 
+### filter_shown_headers()
+
+import { Since } from '@site/src/components';
+
+<Since version="4.3" />
+
+This method adds values to _shownonlyelements array to decide whether a header should be shown or hidden.
+
+```php
+public function filter_shown_headers(array $shownonly): void {
+    $this->_shownonlyelements = [];
+    if (empty($shownonly)) {
+        return;
+    }
+    foreach ($shownonly as $headername) {
+        $element = $this->getElement($headername);
+        if ($element->getType() == 'header') {
+            $this->_shownonlyelements[] = $headername;
+            $this->setExpanded($headername);
+        }
+    }
+}
+```
+
 ### Other features
 
 In some cases you may want to [group elements](https://docs.moodle.org/dev/lib/formslib.php_Form_Definition#Use_Fieldsets_to_group_Form_Elements) into collections.