Issue #2486989 by metzlerd, jhodgdon: Create Documentation For Container Form Elements

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

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

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

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

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

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

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