R1:fe774e68e255

Distinguish between ported-forward and ported-backward comments

Summary:
Ref T7447. This might be overkill, but I want to over-explain things until we have more confidence that this is rarely confusing.

NOTE: I'm playing it a bit fast and loose with `setIsGhost()` (passing a dictionary) because making API changes requires changing the interface and Diffusion, which is a pain. I'll clean this up at the end once the interface is more final. This is well-contained for now.

Test Plan:
- Viewed "base vs 2"…