DOC: add one more column may better
#### Location of the documentation
https://pandas.pydata.org/pandas-docs/stable/reference/api/pandas.DataFrame.cumsum.html
[this should provide the location of the documentation, e.g. "pandas.read_csv" or the URL of the documentation, e.g. "https://dev.pandas.io/docs/reference/api/pandas.read_csv.html"]

**Note**: You can check the latest versions of the docs on `master` [here](https://pandas.pydata.org/docs/dev/).

#### Documentation problem

[this should provide a description of what documentation you believe needs to be fixed/improved]

#### Suggested fix for documentation
in the example shows how pandas.DataFrame.cumsum works, i think add one more column in the example df.cumsum(axis=1) will make thing more clear. Just like df.cumsum(axis=0),which have 3 row in the example
[this should explain the suggested fix and **why** it's better than the existing documentation]
