[1.8.x] Fixed #25515 -- Documented the return value of BaseCommand.handle()

Backport of 3f766d44 from master

docs/howto/custom-management-commands.txt

@@ -366,6 +366,9 @@ the :meth:`~BaseCommand.handle` method must be implemented.
 
     The actual logic of the command. Subclasses must implement this method.
 
+    It may return a Unicode string which will be printed to ``stdout`` (wrapped
+    by ``BEGIN;`` and ``COMMIT;`` if :attr:`output_transaction` is ``True``).
+
 .. method:: BaseCommand.check(app_configs=None, tags=None, display_num_errors=False)
 
     .. versionadded:: 1.7