DOC: combine with None at same position in both dataframes
#### Location of the documentation

https://pandas.pydata.org/pandas-docs/stable/reference/api/pandas.DataFrame.combine.html

#### Documentation problem
The fourth example states:
> However, if the same element in both dataframes is None, that None is preserved

In the code sample to illustrate this, however, `None`  is **not** preserved:
```
df1 = pd.DataFrame({'A': [0, 0], 'B': [None, 4]})
df2 = pd.DataFrame({'A': [1, 1], 'B': [None, 3]})
df1.combine(df2, take_smaller, fill_value=-5)
    A    B
0  0 -5.0
1  0  3.0
```
#### Suggested fix for documentation

If the actual result is the intended behavior, this code sample should be removed. If `None` should indeed be preserved (but why should it?) then this should be filed as a bug.