diff --git a/core/lib/Drupal/Core/Plugin/PluginFormInterface.php b/core/lib/Drupal/Core/Plugin/PluginFormInterface.php
index 9ca47e0..8af54e2 100644
--- a/core/lib/Drupal/Core/Plugin/PluginFormInterface.php
+++ b/core/lib/Drupal/Core/Plugin/PluginFormInterface.php
@@ -9,6 +9,14 @@
 
 /**
  * Provides an interface for a plugin that contains a form.
+ *
+ * Plugin forms are usually contained by other forms. In order to know where the
+ * plugin form is located in the parent form, let buildConfigurationForm()
+ * return a form element that has a #process callback and build the rest of the
+ * form in the callback. By the time the callback is executed, the element's
+ * #parents and #array_parents properties will have been set by the form API.
+ * For more documentation on those properties, see
+ * https://api.drupal.org/api/drupal/developer!topics!forms_api_reference.html/8.
  */
 interface PluginFormInterface {