TST: add tests for Timestamp.toordinal/fromordinal
- [x] follow-up for #13593
- [x] tests added / passed
- [x] passes `git diff upstream/master | flake8 --diff`

added tests and fixed missed doc change.

## [Current coverage](https://codecov.io/gh/pydata/pandas/pull/13610?src=pr) is **84.38%**

> Merging [#13610](https://codecov.io/gh/pydata/pandas/pull/13610?src=pr) into [master](https://codecov.io/gh/pydata/pandas/branch/master?src=pr) will not change coverage

``` diff
@@             master     #13610   diff @@
==========================================
  Files           142        142          
  Lines         51225      51225          
  Methods           0          0          
  Messages          0          0          
  Branches          0          0          
==========================================
  Hits          43225      43225          
  Misses         8000       8000          
  Partials          0          0          
```

> Powered by [Codecov](https://codecov.io?src=pr). Last updated by [20de266...28f8d41](https://codecov.io/gh/pydata/pandas/compare/20de2661c8eff66e465248cbe28062eae0e0e3bb...28f8d411aac7cf091ce968b5693825f9f82d896c?src=pr)
 thanks!
