Liquid Nodes section is out-of-date

The Liquid Nodes section in chapter 6 is now out of date. It was obviously written back when liquid properties were still strongly coupled, e.g. you can only use the liquid drawtypes if the node also behaved like a liquid.

But now, liquid drawtypes and liquid behaviors are now completely separate and modders and combine them as they like. You can now use the liquid drawtypes just fine and ignore all the physical liquid properties like liquidtype, liquid_viscosity, etc.

I think the section is generally written poorly anyway as it doesn't even really explain how the liquid drawtype actually works. And no, "just look at Minetest Game" is not an explanation. So the whole section probably needs a full rewrite anyway.