DOC: Dataframe.apply does not always return a Series when reduce=True
#### Code Sample, a copy-pastable example if possible

```python
pd.DataFrame({'col_1':['val_1','val_2','val_3']}).apply(lambda row: [1], axis=1, reduce=True)
```
#### Problem description
 Per the documentation, calling Dataframe.apply w/ reduce=True should always produce a Series.  However, if the applied function returns list values Dataframe.apply returns a Dataframe, not a Series.

#### Expected Output

When called with reduce=True, I expect Dataframe.apply to produce a Series where each element is a list, not a Dataframe.

#### Output of ``pd.show_versions()``

0.18.1
