It's a way of criticizing something in a humorous way.
Jim and Huck discuss King Solomon satirically, criticizing and stimulating humor. This provokes the reader's reflection, making him think about the subject more lightly and understand the ridiculed subject in a light way.
Limbo is where unchristian and pagans go in the after life while the second circle(which i'm assuming you're talking about) is where sinners of Lust go.
