NEW must nocht be followed by an underscore (#5598)

* [TASK] Clarify datahandler new uid requirements

* Update Documentation/ApiOverview/DataHandler/Database/Index.rst

* Update Index.rst

---------

Co-authored-by: Lina Wolf <[email protected]>

Author: pschriner

Documentation/ApiOverview/DataHandler/Database/Index.rst

@@ -417,6 +417,11 @@ Description of keywords in syntax:
     ...) or generate them using
     :php:`\TYPO3\CMS\Core\Utility\StringUtility::getUniqueId('NEW')`.
 
+..  caution::
+
+    If you supply your own string `NEW` must not be followed by an underscore.
+    The occurance of an underscore implies a reference to a record in a table.
+
 
 ..  confval:: fieldname
     :name: datahandler-data-fieldname