Add some documentation
Add some documentation

While going through the code I thought I'd add some documentation in places where I had to figure out what it was doing.
And in some places I didn't add any because I didn't think of it or I was lazy.

Contributes to issue CURA-4513.

Ultimaker/CuraEngine,Ultimaker/CuraEngine,ROBO3D/CuraEngine,ROBO3D/CuraEngine,ROBO3D/CuraEngine