Let formatter summaries return arrays instead of strings

Agreed.
@sun even advocated for a render array.

Minor:

+++ b/core/modules/field/tests/modules/field_test/field_test.moduleundefined
@@ -272,7 +272,8 @@ function field_test_field_formatter_settings_form_alter(&$element, &$form_state,
+  $summary[] = '<br />field_test_field_formatter_settings_summary_alter';

The <br /> is not needed anymore ?

Other than that, looks good to me :-)

Yay, thanks !

Great!

+++ b/core/modules/field_ui/field_ui.api.phpundefined
@@ -93,10 +93,11 @@ function hook_field_instance_settings_form($field, $instance, $form_state) {
 /**
- * Alter the formatter settings form.
+ * Alter the formatter settings form invoked from
+ * \Drupal\field_ui\DisplayOverView.

Like this..

/**
 * Alters the formatter settings form.
...
...
 *
 * @see \Drupal\field_ui\DisplayOverView

And do the same for the other similar cases...

Committed 30e1599 and pushed to 8.x. Thanks!

Excellent, we don't need more!

Reviewed and refined the change notice on irc, looks good to me now.