Fill in "Changes in type checking/inference" sections

[odersky]

Right now the reference contains two sections under "Changed Features"

• Changes in Type Checking
• Changes in Type Inference

The first is empty and the second just points to a video of a conference talk by @smarter.

We should either drop these sections or fill them with material. If we fill in material then the topic of target-typed new expressions should go into Changes in Type Inference.

[abgruszecki]

@odersky @b-studios do we want to keep this issue around?

[ckipp01]

I still think we should keep this open as having this section filled out would be nice (unless we do have it documented somewhere else. I did create #17494 just to remove that empty page, which really shouldn't be live.