Add reference of DataFrame.rename_axis and Series.rename_axis to api.rst
- [x] closes #13642
- [ ] tests added / passed
- [ ] passes `git diff upstream/master | flake8 --diff`
- [ ] whatsnew entry

## [Current coverage](https://codecov.io/gh/pydata/pandas/pull/13678?src=pr) is **84.38%**

> Merging [#13678](https://codecov.io/gh/pydata/pandas/pull/13678?src=pr) into [master](https://codecov.io/gh/pydata/pandas/branch/master?src=pr) will increase coverage by **<.01%**

``` diff
@@             master     #13678   diff @@
==========================================
  Files           142        142          
  Lines         51223      51223          
  Methods           0          0          
  Messages          0          0          
  Branches          0          0          
==========================================
+ Hits          43223      43224     +1   
+ Misses         8000       7999     -1   
  Partials          0          0          
```

> Powered by [Codecov](https://codecov.io?src=pr). Last updated by [d7c028d...0fda07f](https://codecov.io/gh/pydata/pandas/compare/d7c028d4965932160fa3b69f56c716b1454c42a5...0fda07f71678be8041542f46d93b194d570640ac?src=pr)
 @sinhrks I added the commit for Series as requested and squashed the commits together. This is my first PR, so feel free to let me know if there's a better way to do this. 
 Thanks. Congrats on your first PR to pandas 🎉 

The changes should show up [here](http://pandas-docs.github.io/pandas-docs-travis/) a bit after [this](https://travis-ci.org/pydata/pandas/builds/145343146) Travis run finishes.
