title: wordpress do_action without add_action and action hook names
body: <p>I kind of understand add_action and then do_action. for example</p>
  
  <pre><code>function mywork()
  {
      echo "display my work";
  }
  add_action('mytag','mywork',10);
  do_action('mytag');
  </code></pre>
  
  <p>this will render "display my work";</p>
  
  <p>But how about just do_action only. 
  for example </p>
  
  <pre><code>function customfunction()
      {
          echo "custom";
      }
  do_action('customfunction');
  </code></pre>
  
  <p>Nothing is rendered.
  I also found some wordpress api hook names. for example "login_form"</p>
  
  <pre><code>do_action(login_form);
  </code></pre>
  
  <p>But nothing happened.</p>