title: What does Module.private_constant do? Is there a way to list only private constants?
body: <p>Starting in Ruby 1.9.3, we can create private constants:</p>
  
  <pre><code>module M
    class C; end
    private_constant :C
  end
  </code></pre>
  
  <p>Is there a good documentation about what this does? Is there a way to get the names of only private constants similar to calling <code>constants</code></p>