title: NumPy 2D array boolean indexing with each axis
body: <p>I created 2D array and I did boolean indexing with 2 bool index arrays.
  first one is for axis 0, next one is for axis 1.</p>
  <p>I expected that values on cross True and True from each axis are selected like Pandas.
  but the result is not.</p>
  <p>I wonder how it works that code below.
  and I want to get the link from official numpy site describing this question.</p>
  <p>Thanks in advance.</p>
  <pre class="lang-py prettyprint-override"><code>a = np.arange(9).reshape(3,3)
  a
  ----------------------------
  array([[0, 1, 2],
         [3, 4, 5],
         [6, 7, 8]])
  </code></pre>
  <pre class="lang-py prettyprint-override"><code>a[ [True, False, True], [True, False, True] ]
  --------------------------
  array([0, 8])
  </code></pre>
  <p>My expectation is <code>[0, 6, 2, 8]</code>.
  (I know how to get the result that I expect.)</p>