title: Doxygen - declare parameter as optional
body: <p>I am documenting a codebase using Doxygen and was wondering if there is a keyword for declaring an argument to a function to be optional.  Something like:</p>
  
  <pre><code>/*!
  \fn int add(int a, int b=0)
  \brief adds two values
  \param a the first operand
  \param \optional b the second operand.  Default is 0
  \return the result
  */
  </code></pre>
  
  <p>It seems like this is something that should exist, but I haven't been able to find it anywhere.  Is there an actual option, or do I just need to make note in the description?</p>