chore: Add note on CompliantDataFrame.aggregate

- Have a feeling this was used more in the past
- E.g. when `select` accepted `*args, **kwds`

Author: dangotbanned

narwhals/_compliant/dataframe.py

@@ -46,6 +46,8 @@ def simple_select(self, *column_names: str) -> Self:
         """`select` where all args are column names."""
         ...
 
+    # NOTE: Can we remove this now?
+    # `DaskLazyFrame` is the only one not the same as `select`
     def aggregate(self, *exprs: Any) -> Self:  # pragma: no cover
         """`select` where all args are aggregations or literals.