DOC: clarify that .ix[] is always label based with integer index
According to the documentation, ix should accept the all the input to iloc. In the following the .ix raises  an KeyError while .iloc work as expected. I suspect the fall back mechanism is broken in this case with integer index. 

``` python
import pandas as pd
import numpy as np

df = pd.Series(np.random.randn(9))
df.iloc[-1]
df.ix[-1] # raise KeyError
```

version: 
pandas: 0.13.1
python: 2.7.6.final.0

EDIT: Changed title from "df.ix[-1] broken" to "DOC: clarify that .ix[] is always label based with integer index"
