DOC: clarify the behavior of the .as_matrix method
When using `df.as_matrix()` method, rows and columns do not render as 1xN or Nx1 matricies, rather as 1xN arrays.

```
In [4]: df.ix['foo']=[5,3]

In [5]: df.ix['bar']=[2,6]

In [6]: df
Out[6]:
     A  B
foo  5  3
bar  2  6

[2 rows x 2 columns]

In [7]: df['A'].as_matrix()
Out[7]: array([ 5.,  2.])

In [8]: df.ix['foo'].as_matrix()
Out[8]: array([ 5.,  3.])
```

Expected :

```
In [9]: np.matrix('5; 2')
Out[9]:
matrix([[5],
        [2]])
```
