DOC: Improvement docstring of DataFrame.rank()
- [X] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`

I have made the changes requested in the pull request #23263 

This is the output of the docstring validation:
```
3 Errors found:
	Parameter "method" description should finish with "."
	Parameter "na_option" description should finish with "."
	The first line of the Returns section should contain only the type, unless multiple values are being returned
```
In the previous pull request I have been told that I could ignore these 3 errors.

Please let me know if I can further improve the docstring.

Thanks a lot