docs(Pane): Fix docstring ordering in pane.split_window (#528)

* docs: Fix docstring ordering of vertical and start_directory in
pane.split_window

* docs: Add environment docstring to pane.split_window

Author: Ngalstyan4

src/libtmux/pane.py

@@ -500,12 +500,14 @@ def split_window(
         ----------
         attach : bool, optional
             Attach / select pane after creation.
-        vertical : bool, optional
-            split vertically
         start_directory : str, optional
             specifies the working directory in which the new pane is created.
+        vertical : bool, optional
+            split vertically
         percent: int, optional
             percentage to occupy with respect to current pane
+        environment: dict, optional
+            Environmental variables for new pane. tmux 3.0+ only. Passthrough to ``-e``.
 
         Notes
         -----