DOC: improve docs for broadcasting behavior
The following result was quite surprising to me:

```python
>>> df = pd.DataFrame({'a': np.arange(3),
                       'b': np.ones(3)})
>>> df + [np.ones(3), np.ones(3)]
                 a                b
0  [1.0, 1.0, 1.0]  [2.0, 2.0, 2.0]
1  [2.0, 2.0, 2.0]  [2.0, 2.0, 2.0]
2  [3.0, 3.0, 3.0]  [2.0, 2.0, 2.0]
```

It looks like if you add a list to a dataframe, pandas [casts it to a one-dimensional Series](https://github.com/pandas-dev/pandas/blob/04db779d4c93d286bb0ab87780a85d50ec490266/pandas/core/ops.py#L1159), where the entries of the series are array objects. The addition is then broadcasted down the columns such that each entry of the result is an array.

This behavior is quite counter-intuitive to me: is it a bug? If not, should it be documented?