DOC: slight change in .cum* function descriptions
closes #13682

I changed the template function to use the _desc_ instead of the _name_ in generic.py.

SIDENOTE: I accidentally committed to a branch with another PR, so I rebased without it and created a new branch with this commit. 

## [Current coverage](https://codecov.io/gh/pydata/pandas/pull/13683?src=pr) is **84.38%**

> Merging [#13683](https://codecov.io/gh/pydata/pandas/pull/13683?src=pr) into [master](https://codecov.io/gh/pydata/pandas/branch/master?src=pr) will increase coverage by **<.01%**

``` diff
@@             master     #13683   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...ada4cda](https://codecov.io/gh/pydata/pandas/compare/d7c028d4965932160fa3b69f56c716b1454c42a5...ada4cda58ce8eac4c55608539c4540e86de8a9ce?src=pr)
 @shawnheide Good catch, thanks a lot!
