Pane(docs[set_title]): Add Returns description to docstring

why: Returns section had type but no description, inconsistent with other
methods like resize_width/resize_height.
what:
- Add description text for the :class:\`Pane\` return type

Author: tony

src/libtmux/pane.py

@@ -842,6 +842,7 @@ def set_title(self, title: str) -> Pane:
         Returns
         -------
         :class:`Pane`
+            The pane instance, for method chaining.
 
         Examples
         --------