In this section:
"Enable edit history tracking and prune after X days:"
You'll notice the following text:
"If enabled, moderators will be able to see historical versions of messages and compare changes between them. Historical data will be pruned after the specified number of days. Use 0 to keep the history for ever."
I'm guessing that should be "forever". Or, for consistency with the first option on the Logging Options page, it could just be made to match that: "Use 0 to keep the records permanently".