DOC: Add description for tz parameter in DatetimeIndex
- [ ] closes #xxxx
- [ ] tests added / passed
- [x] passes `black pandas`
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [ ] whatsnew entry

@nsiregar Should we have `time zone` or `timezone`?  @za it's `time zone` in dictionary. @nsiregar I think CI error is unrelated - can you merge master and repush to be sure? @nsiregar can you please merge master into your branch, and address the remaining comment? Thanks! Closing as stale.