Add a case when CType migration has not been done

Author: Oktopuce

Documentation/Exceptions/1540246570.rst

@@ -126,3 +126,15 @@ The error does not occur when you use the Fluid lanuage tag with a path:
 ..  code-block:: xml
 
     <f:translate key = "LLL:EXT:yourExtension/Resources/Private/Language/locallang.xml:Button_Helpful" />
+
+TYPO3 12/13 - General Plugin: No Content Object definition found at TypoScript object path "tt_content.list.20
+==============================================================================================================
+
+This exception could happen because a plugin were registered before using the :php:`list` content element and the plugin subtype :php:`list_type` and are now registered as :php:`ExtensionUtility::PLUGIN_TYPE_CONTENT_ELEMENT`.
+
+Have a look at https://docs.typo3.org/c/typo3/cms-core/main/en-us/Changelog/13.4/Deprecation-105076-PluginContentElementAndPluginSubTypes.html
+
+Either an Upgrade Wizard was not executed for those plugins or may be you need to create one.
+
+If Upgrade Wizard creation is required, you can have a look in TYPO3 core for example, search Upgrade Wizard extending :php:`AbstractListTypeToCTypeUpdate`, like :php:`final class IndexedSearchCTypeMigration extends AbstractListTypeToCTypeUpdate`.
+