title: Why the DataFrame.nlargest does not order a result?
body: <p>Here is an example from Pandas documentation v0.22.0:</p>
  
  <pre><code>In [330]: df = pd.DataFrame({'a': [-2, -1, 1, 10, 8, 11, -1],
                               'b': list('abdceff'),
                               'c': [1.0, 2.0, 4.0, 3.2, np.nan, 3.0, 4.0]})
  
  In [332]: df.nlargest(5, ['a', 'c'])
  Out[332]: 
       a  b    c
   6  -1  f  4.0
   5  11  f  3.0
   3  10  c  3.2
   4   8  e  NaN
   2   1  d  4.0
  </code></pre>
  
  <p>Why the result is not ordered first by 'a' and then by 'c'? </p>
  
  <p>At the same time the applying <code>nlargest()</code> with single column always order the result:</p>
  
  <pre><code>df.nlargest(5,['a'])
  Out[110]: 
       a  b    c
  5  11.0  f  3.0
  3  10.0  c  3.2
  4   8.0  e  NaN
  2   1.0  d  4.0
  1  -1.0  b  2.0
  </code></pre>