dwindle: document swapsplit (#471)

See-Also: hyprwm/Hyprland#4702

pages/Configuring/Dwindle-Layout.md

@@ -38,6 +38,7 @@ Dispatcher `layoutmsg` params:
 | param | description | args |
 |---|---|---|
 | togglesplit | toggles the split (top/side) of the current window. `preserve_split` must be enabled for toggling to work. | none |
+| swapsplit | swaps the two halves of the split of the current window. | none |
 | preselect | A onetime override for the split direction. (valid for the next window to be opened, only works on tiled windows) | direction |
 
 e.g.: