DOC: Add Clarification to Groupby.Filter for DataFrames
All of the examples in the docs for [Groupby.Filter](http://pandas.pydata.org/pandas-docs/dev/groupby.html#filtration) showcase examples in which the GroupBy collapses into a series, and is evaluated in a single dimension.

I'd like to suggest adding (or amending the third example) to include a case in which the initial dataframe has > 2 columns, and thereby the GroupBy retains two dimensions.

An additional example might look like:

```
In [86]: df3 = DataFrame({'A': np.arange(8), 'B': list('aabbbbcc'), 'C': np.arange(8)})

In [86]: df3.groupby('B').filter(lambda x: len(x['C']) > 2)

   A  B  C
2  2  b  2
3  3  b  3
4  4  b  4
5  5  b  5
```

I suggest this because I struggled with the amusing errors generated by not specifying a column for most of the afternoon.
