title: in dlib python API document, what does "->None" mean?
body: <p>In dlib Python API document I see these contents(ignore the colors, I don't know why..),<br />
  (from <a href="http://dlib.net/python/index.html#dlib.rectangles" rel="nofollow noreferrer">http://dlib.net/python/index.html#dlib.rectangles</a>)</p>
  <pre><code>class dlib.chip_details
  WHAT THIS OBJECT REPRESENTS This object describes where an image chip is to be extracted from within another image. In particular, it specifies that the image chip is contained within the rectangle self.rect and that prior to extraction the image should be rotated counter-clockwise by self.angle radians. Finally, the extracted chip should have self.rows rows and self.cols columns in it regardless of the shape of self.rect. This means that the extracted chip will be stretched to fit via bilinear interpolation when necessary.
  
  __init__(*args, **kwargs)
  Overloaded function.
  
  __init__(self: dlib.chip_details, rect: dlib.drectangle) -&gt; None
  __init__(self: dlib.chip_details, rect: dlib.rectangle) -&gt; None
  ensures
  self.rect == rect_
  self.angle == 0
  self.rows == rect.height()
  self.cols == rect.width()
  __init__(self: dlib.chip_details, rect: dlib.drectangle, size: int) -&gt; None
  __init__(self: dlib.chip_details, rect: dlib.rectangle, size: int) -&gt; None
  ensures
  ...
  </code></pre>
  <p>What does &quot;-&gt;None&quot; mean?</p>