general dataframe combine doc


#### Problem description


https://github.com/pandas-dev/pandas/blob/master/doc/source/getting_started/basics.rst#general-dataframe-combine


```python
def combiner(x, y):
    np.where(pd.isna(x), y, x)
df1.combine(df2, combiner)
```
the code above should be:

```python
def combiner(x, y):
    return np.where(pd.isna(x), y, x)
df1.combine(df2, combiner)
```
