DOC: Fix docstrings of Excel related functions
The next Excel related functions contain many errors as reported by `scripts/validate_docstrings.py`:
- pandas.read_excel
- pandas.io.formats.style.Styler.to_excel
- Series.to_excel
- DataFrame.to_excel

We should fix them, following our standards: https://pandas.pydata.org/pandas-docs/stable/contributing_docstring.html

The validation script should not report errors after they are fixed.