"rebake old posts count" option is a bit confusing

Sure happy to update the description.

How does this sound @codinghorror

Maximum number of posts to be automatically rebaked after major changes to the Discourse rendering engine every 15 minutes.

Maximum number of old-version posts to be re-rendered to HTML every 15 minutes. Will only impact major rendering engine upgrades. (once every few years)

Hard to know what fidelity the description here needs?