Keep an eye on 6.e.PREVIEW

[JJ]

The problem

The pragma is going to be available from 2019.06, and we need to keep an eye on features releases under the preview to incorporate them in the documentation.

Suggestions

Let's keep this issue open, and list below what we will incorporate into the documentation.

Don't close it until 6.e.PREVIEW becomes 6.e (or all features have been incorporated).

[coke]

This is covered by the checklist tickets created for each release (new things that may or may not be scoped to 6.e), and by #302