diff --git a/core/modules/system/system.install b/core/modules/system/system.install
index 2fe2533..3c8332e 100644
--- a/core/modules/system/system.install
+++ b/core/modules/system/system.install
@@ -1993,7 +1993,7 @@ function system_update_8402() {
   $definition_update_manager = \Drupal::entityDefinitionUpdateManager();
 
   // Clear the cached entity type definitions so we get the new
-  // 'revision_translation_affected' entity key
+  // 'revision_translation_affected' entity key.
   \Drupal::entityTypeManager()->clearCachedDefinitions();
 
   // Get a list of revisionable and translatable entity types.
@@ -2015,6 +2015,9 @@ function system_update_8402() {
         ->setReadOnly(TRUE)
         ->setRevisionable(TRUE)
         ->setTranslatable(TRUE)
+        // Mark all pre-existing revisions as affected in order to be consistent
+        // with the previous API return value: if the field was not defined the
+        // value returned was always TRUE.
         ->setInitialValue(TRUE);
 
       $definition_update_manager