title: Show choices of option in -h. When using and optional command line argument in python with argparse
body: <p>So I want to start my program with an optional argument which has pre-defined choices where the options show up in the -h screen. So that a user knows which option are available for selection.</p>
  
  <pre><code>parser = argparse.ArgumentParser(description="This is a program which creates a prediction model for the boston housing dataset.")
  
  parser.add_argument("Model", help="Choose which model you want to use for prediction.",
  type=str, choices=["linear_regression", "polynomial_regression"])
  
  parser.add_argument("--fd", metavar="FEEDBACK", help="Set how much feedback you want.",
  type=str, choices=["full", "immediate", "weak"])
  
  args = parser.parse_args()
  </code></pre>
  
  <p>The problem is when I start the program with -h I don't see the available choices for --fd. How can I change that so that the user can see the available choices for --fd like with the Model choices shown in the picture?</p>
  
  <p><a href="https://i.stack.imgur.com/9MWkI.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/9MWkI.png" alt="enter image description here"></a></p>