DOC: minor doc fix for Series.append; indices can overlap
Old docstring seemed to be incorrect; overlapping indices are supported e.g.

``` python
>>> import pandas as pd
>>> x = pd.Series(['A', 'B', 'C'])
>>> y = pd.Series(['D', 'E', 'F'])
>>> x.append(y)
0    A
1    B
2    C
0    D
1    E
2    F
dtype: object
```
