Issue #2779999 by shashikant_chauhan, guilhermevp, dww, joachim, xjm: Document checkboxes and radios element can have individual descriptions

(cherry picked from commit 46cc0e0fcb)

core/lib/Drupal/Core/Render/Element/Checkboxes.php

@@ -23,6 +23,12 @@ use Drupal\Core\Form\FormStateInterface;
  * );
  * @endcode
  *
+ * Element properties may be set on single option items as follows.
+ *
+ * @code
+ * $form['high_school']['tests_taken']['SAT']['#description'] = $this->t('Description for the SAT option.');
+ * @endcode
+ *
  * @see \Drupal\Core\Render\Element\Radios
  * @see \Drupal\Core\Render\Element\Checkbox
  *

core/lib/Drupal/Core/Render/Element/Radios.php

@@ -22,6 +22,12 @@ use Drupal\Component\Utility\Html as HtmlUtility;
  * );
  * @endcode
  *
+ * Element properties may be set on single option items as follows.
+ *
+ * @code
+ * $form['settings']['active'][0]['#description'] = $this->t('Description for the Closed option.');
+ * @endcode
+ *
  * @see \Drupal\Core\Render\Element\Checkboxes
  * @see \Drupal\Core\Render\Element\Radio
  * @see \Drupal\Core\Render\Element\Select