DOC: DataFrame on df.nunique causing confusion
#### https://pandas.pydata.org/docs/reference/api/pandas.DataFrame.nunique.html

#### Documentation problem

The above documentation provides an example using `pd.DataFrame({'A': [1, 2, 3], 'B': [1, 1, 1]})` upon which `df.nunique()` returns 
```
A    3
B    1
``` 
This example might be a bit misleading as there are `3` and `1` present in dataframe as column elements and cause confusion. 
#### Suggested fix for documentation

I would like to  suggest using something like `pd.DataFrame({'A': [5, 6, 7], 'B': [5, 1, 1]})` upon which `df.nunique()` returns 
```
A 3
B 2
```
as this examples serves the purpose of `nunique` better as its clear that it counts the number of unique elements in column and does not return anything resembling any column element.

Furthermore, I would like to extend the example dataframe to `df.nunique(axis=1)` and match corresponding values with new dataframe as : 
```
df.nunique(axis=1)
0    1
1    2
2    2
dtype: int64
```

Does this change need to be documented in `whatsnew`?