DOC: MultiIndex Indexing Using Slices
I have the following `DataFrame`:

```
    t1  t2
    b1  b1
i1   1   1
```

which I can index like this:

```
In [139]: df["t1"]
Out[139]: 
    b1
i1   1

In [140]: df["t1","b1"]
Out[140]: 
i1    1
Name: (t1, b1), dtype: int64

In [141]: df["t1"]["b1"]
Out[141]: 
i1    1
Name: b1, dtype: int64
```

I would expect this to work, but ti does not:

```
In [142]: df[:]["b1"]
KeyError: 'b1'
```

as well as this (which also raises an unhelpful error message):

```
In [143]: df[:,"b1"]
TypeError: 
```
