A fan asked on the official Blizzard forums about more details for damage displays.

Bashiok had the following answer: 

Bashiok has answered a interesting question about the Witch Doctors skill Soul Harvest.

Question:

While looking into the Witch Doctor, he has a skilled called Soul Harvest. With this skill he basically has an AoE attack that kills the monsters around him, releasing their souls which he then “absorbs”.