RNA: UILayout.split(..) 'percentage' to 'factor'

Misleading name since it's between 0..1.

Use as a keyword argument to prepare for keyword only args.

Also document that leaving unset has special behavior.

release/scripts/modules/bpyml.py

@@ -182,7 +182,7 @@ if __name__ == "__main__":
                     prop(data='context.scene.render', property='stamp_font_size', text='Font Size')
                 ]
             ],
-            split(percentage=0.2)[
+            split(factor=0.2)[
                 prop(data='context.scene.render', property='use_stamp_note', text='Note'),
                 row()[
                     active(expr='context.scene.render.use_stamp_note'),