DOC: split docstring into multiple lines in excel.py
- [x] closes #13967 
- [x] tests added / passed
- [x] passes `git diff upstream/master | flake8 --diff`
- [ ] whatsnew entry 

A follow-up on #14030.
Not sure whether this is the good way to do it - this seems too verbose.
I will add it to `parsers.py` or move it to `common.py` once it's done.

I also thought about moving the function to `Appender` to fix the length problem for good if necessary.
