Update to_dataframe doc to match current behavior (#9662)

Author: mgunyho

xarray/core/dataset.py

@@ -7371,7 +7371,7 @@ def to_dataframe(self, dim_order: Sequence[Hashable] | None = None) -> pd.DataFr
             dataframe.
 
             If provided, must include all dimensions of this dataset. By
-            default, dimensions are sorted alphabetically.
+            default, dimensions are in the same order as in `Dataset.sizes`.
 
         Returns
         -------