DOC: Improved links between expanding and cum* (GH12651)
- [x] closes #12651 
- [x] passes `git diff upstream/master | flake8 --diff`

## [Current coverage](https://codecov.io/gh/pydata/pandas/pull/14098?src=pr) is 85.26% (diff: 100%)

> Merging [#14098](https://codecov.io/gh/pydata/pandas/pull/14098?src=pr) into [master](https://codecov.io/gh/pydata/pandas/branch/master?src=pr) will decrease coverage by **<.01%**

``` diff
@@             master     #14098   diff @@
==========================================
  Files           139        139          
  Lines         50490      50502    +12   
  Methods           0          0          
  Messages          0          0          
  Branches          0          0          
==========================================
+ Hits          43053      43063    +10   
- Misses         7437       7439     +2   
  Partials          0          0          
```

> Powered by [Codecov](https://codecov.io?src=pr). Last update [670435a...4427e28](https://codecov.io/gh/pydata/pandas/compare/670435ab346d6b35e65b3b1de2b56d850ff25cc6...4427e28a0f1ab35eb2acf5ed27b469a1e16a1b01?src=pr)
 can you update
 can you rebase and update this?
 @adrian-stepien Thanks! (adapted it a little bit for @jreback's comments)