API/DOC: unclear scope of index.asof
From http://stackoverflow.com/questions/26918045/python-pandas-typeerror-cannot-compare-type-timestamp-with-type-float

The docstring of `Index.asof` says:

```
In [73]: pd.Index.asof?
Type:        instancemethod
String form: <unbound method Index.asof>
File:        c:\anaconda\lib\site-packages\pandas\core\index.py
Definition:  pd.Index.asof(self, label)
Docstring:
For a sorted index, return the most recent label up to and including
the passed label. Return NaN if not found
```

while `asof` only works for a time series.
- Clarify the docs that `asof` only works for `DatetimeIndex` (and PeriodIndex?)
- Why is this actually restricted to timestamps? This functions seems also possibly useful to me for other index types? Something like
  
  ```
  In [74]: idx = pd.Index([1,2,3,8,9])
  
  In [75]: idx.asof(5)    # gives now TypeError
  3
  ```
  
  Instead of having to do:
  
  ```
  In [78]: idx[idx.searchsorted(5) - 1]
  Out[78]: 3
  ```
