[FEATURE] Document cli backend login

Taken from
https://stackoverflow.com/questions/54881504/how-to-create-a-file-relation-in-typo3-v9-in-a-cli-command-from-symfony

Documentation/ApiOverview/CommandControllers/Index.rst

@@ -97,6 +97,19 @@ Create a custom command
 See the :ref:`Tutorial: Create a console command <console-command-tutorial>`
 for details on how to create commands.
 
+DataHandler usage
+=================
+Using the :ref:`DataHandler <tce-database-basics>` in a cli command will
+fail with the following error:
+
+ [1.2.1]: Attempt to modify table "pages" without permission
+
+Log in as the `_cli` backend user first to circumvent that problem:
+
+.. code-block:: php
+
+   $GLOBALS['BE_USER']->backendCheckLogin();
+
 Read more
 ==========