Revert "Cut section on incremental tasks out." - we are announcing it.

This reverts commit a08d68a3798eaed1c256ffe410e0eac981a12e4a.
Tweaks to the release notes on incremental tasks.

This includes a fat disclaimer that it's very likely to change in the near future.
Add a disclaimer to the incremental tasks user guide chapter to the effect that it is very likely to change in upcoming releases.