How to show forum description under forum title

Where exactly? On forum home it's already there. When you click and open a forum with topic list it's not there. We'll add it in next version, currently there is no option.

oh.. that's where I needed it.  not the forum home, but the forum with the topic list.  thank you.  i look forward to the update.

As per your wish you can edit the forum and put the description in the content.  
dashboard>forums>all forums

You can also use theme by theme canon and can create a forum and within the forum, placed a topic.