Update docs for groupby based on the discussion in data-apis#102

Author: kkraus14

spec/API_specification/dataframe_api/dataframe_object.py

@@ -11,9 +11,27 @@
 
 
 class DataFrame:
-    def groupby(self, keys: list[str], /) -> GroupBy:
+    def groupby(self, keys: Sequence[str], /) -> GroupBy:
         """
         Group the DataFrame by the given columns.
+
+        Parameters
+        ----------
+        keys : Sequence[str]
+
+        Returns
+        -------
+        GroupBy
+
+        Raises
+        ------
+        KeyError
+            If any of the requested keys are not present.
+
+        Notes
+        -----
+        The order of the keys and the order of rows within each group is not
+        guaranteed and is implementation defined.
         """
         ...