DOC: Corrects 'reindex_axis' docstring
The docstring included a 'tolerance' parameter which is missing from the source. This has been removed.

The docstring did not describe the 'fill_value' parameter. This has been added, mostly reusing the wording in the summary part of the existing docstring.

- [x] closes #23960
- [ ] tests added / passed
- [ ] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [ ] whatsnew entry
