AP-25288: Improve docstring of _generate_description

Co-authored-by: Copilot <[email protected]>

Author: AtR1an

org.knime.python3.nodes/src/main/python/knime/extension/parameter.py

@@ -1806,6 +1806,12 @@ def _generate_description(self, visible_options=None):
             from self._enum are included. If provided, only these members appear
             in the description.
 
+        Returns
+        -------
+        str
+            A formatted description string containing the available options,
+            optionally restricted to the provided ``visible_options``.
+
         Returns
         -------
         str