DOC: Fixing GL08 errors for pandas.ExcelFile.sheet_names and pandas.MultiIndex.codes
Resolved GL08 errors for the following:

1. scripts/validate_docstrings.py --format=actions --errors=GL08 pandas.ExcelFile.sheet_names
2. scripts/validate_docstrings.py --format=actions --errors=GL08 pandas.MultiIndex.codes

Ref: #57443 

- [ ] closes #xxxx (Replace xxxx with the GitHub issue number)
- [ ] [Tests added and passed](https://pandas.pydata.org/pandas-docs/dev/development/contributing_codebase.html#writing-tests) if fixing a bug or adding a new feature
- [x] All [code checks passed](https://pandas.pydata.org/pandas-docs/dev/development/contributing_codebase.html#pre-commit).
- [ ] Added [type annotations](https://pandas.pydata.org/pandas-docs/dev/development/contributing_codebase.html#type-hints) to new arguments/methods/functions.
- [ ] Added an entry in the latest `doc/source/whatsnew/vX.X.X.rst` file if fixing a bug or adding a new feature.

Thanks @thomasdamcevski and @bergnerjonas 