Update PCC guide with new engine-related information

[hexaflexahexagon]

The parallax-corrected cubemaps guide is now slightly out of date as a few things have changed since we got full engine access and were able to clean up the implementation some.

https://docs.momentum-mod.org/shader/parallax-corrected-cubemaps/

A small excerpt of the info is pictured below from Gocnak but it would be ideal if a game developer could jot down a quick list of all the things that need updating just to confirm.