Improve document for **kwargs argument of pandas.Series.to_markdown (#34616)

Author: nuka137

pandas/core/series.py

@@ -1422,6 +1422,21 @@ def to_string(
         |  1 | pig      |
         |  2 | dog      |
         |  3 | quetzal  |
+
+        Output markdown with a tabulate option.
+
+        >>> print(s.to_markdown(tablefmt="grid"))
+        +----+----------+
+        |    | animal   |
+        +====+==========+
+        |  0 | elk      |
+        +----+----------+
+        |  1 | pig      |
+        +----+----------+
+        |  2 | dog      |
+        +----+----------+
+        |  3 | quetzal  |
+        +----+----------+
         """
     )
     @Substitution(klass="Series")