title: Form field description in django admin
body: <p>How to add hint for the form field in django admin like in next example?</p>
  
  <p><img src="https://i.stack.imgur.com/nciDr.png" alt="form field description in django admin"></p>
  
  <p>(here: <strong>URL</strong> and <strong>Content</strong> descriptions are shown with gray color under field)</p>