title: can't find swift darwin documentation
body: <p>I'm playing around with swift and have the following simple code </p>
  
  <p><a href="https://i.stack.imgur.com/wBSuE.png" rel="noreferrer"><img src="https://i.stack.imgur.com/wBSuE.png" alt="enter image description here"></a></p>
  
  <p>I want to check how "sqrt" function is implemented, so I tried to click on "Darwin.C.math" link but nothing happened. I googled "Swift darwin apple documentation" but nothing about "Darwin" came up. So could someone please tell me, how I can find the documentation on the "sqrt" function?</p>
  
  <p>In java, when I click on a class name or a method name, the source code file will appear, which include all the details on how the method and class is implemented. But I can't seem to do the same with swift/xcode. </p>