diff --git a/core/lib/Drupal/Core/Entity/EntityFormBuilderInterface.php b/core/lib/Drupal/Core/Entity/EntityFormBuilderInterface.php
index fc3d1a5..ddec258 100644
--- a/core/lib/Drupal/Core/Entity/EntityFormBuilderInterface.php
+++ b/core/lib/Drupal/Core/Entity/EntityFormBuilderInterface.php
@@ -19,7 +19,11 @@
    *   The entity to be created or edited.
    * @param string $operation
    *   (optional) The operation identifying the form variation to be returned.
-   *   Defaults to 'default'.
+   *   Defaults to 'default'. This is typically used in routing:
+   *   @code
+   *   _entity_form: node.book_outline
+   *   @endcode
+   *   where book.outline is the value of $operation.
    * @param array $form_state
    *   (optional) An associative array containing the current state of the form.
    *   Use this to pass additional information to the form, such as the
diff --git a/core/lib/Drupal/Core/Entity/EntityTypeInterface.php b/core/lib/Drupal/Core/Entity/EntityTypeInterface.php
index 857d3e7..decb4be 100644
--- a/core/lib/Drupal/Core/Entity/EntityTypeInterface.php
+++ b/core/lib/Drupal/Core/Entity/EntityTypeInterface.php
@@ -229,6 +229,8 @@ public function setStorageClass($class);
    *
    * @return string
    *   The class for this operation's form for this entity type.
+   *
+   * @see \Drupal\Core\Entity\\EntityFormBuilderInterface
    */
   public function getFormClass($operation);
 
@@ -238,9 +240,12 @@ public function getFormClass($operation);
    * @param string $operation
    *   The operation to use this form class for.
    * @param string $class
-   *   The form class to use for the operation.
+   *   The form class implementing
+   *   \Drupal\Core\Entity\EntityFormControllerInterface.
    *
    * @return static
+   *
+   * @see \Drupal\Core\Entity\\EntityFormBuilderInterface
    */
   public function setFormClass($operation, $class);