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
```

This would raise if `verify_integrity=True` though, seems like what that was meant for. 

```
In [6]: x.append(y, verify_integrity=True)
ValueError: Indexes have overlapping values: [0, 1, 2]
```

Perhaps a good idea to do this?

```
 The indexes must not overlap if verify_integrity is True
```
 The documentation of `verify_integrity` already says this pretty clearly. I'm not sure the information needs to be repeated in the general description. For what it's worth, [`DataFrame.append`](https://github.com/pydata/pandas/blob/master/pandas/core/frame.py#L3886) and [`pd.concat`](https://github.com/pydata/pandas/blob/master/pandas/tools/merge.py#L700) both have the same behavior, and don't mention anything about duplicate indices in the general description.
 I think this looks good!
 @jakevdp ah I see, yeah that makes sense
 @jakevdp Thanks!
