add to the doc-string a versionadded tag and add Series.to_excel to api.rst

Author: s-celles

doc/source/api.rst

@@ -692,6 +692,7 @@ Serialization / IO / Conversion
    Series.to_pickle
    Series.to_csv
    Series.to_dict
+   Series.to_excel
    Series.to_frame
    Series.to_xarray
    Series.to_hdf

pandas/core/generic.py

@@ -1019,6 +1019,8 @@ def __setstate__(self, state):
     _shared_docs['to_excel'] = """
     Write %(klass)s to a excel sheet
 
+    .. versionadded:: 0.20.0
+    
     Parameters
     ----------
     excel_writer : string or ExcelWriter object