Issue #2589805 by quietone, chx, benjy: Add documentation on how to find the menu migrations

core/modules/menu_link_content/migration_templates/menu_links.yml

@@ -12,6 +12,7 @@ process:
   menu_name:
     -
       plugin: migration
+      # The menu migration is in the system module.
       migration: menu
       source: menu_name
     -

core/modules/menu_ui/migration_templates/menu_settings.yml

+# The menu migration is in the system module and the menu_links migration is in the menu_link_content module.
 id: menu_settings
 label: Menu UI configuration
 migration_tags:

core/modules/system/migration_templates/menu.yml

+# The menu_settings migration is in the menu_ui module.
 id: menu
 label: Menus
 migration_tags: