Improve documentation of terrain types

[BlackYps]

I had a closer look at the terrain types while I was working with the map editor and noticed that the documentation what they actually do is a bit lacking, so we should fix that.

I tried to understand how types that differ from the default are handled, but I have not fully figured it out. Apparently the game sometimes falls back onto the default terrain type if it can't find certain effects, but it also seems possible to remove default effects. @Hdt80bro you had a closer look at this all a while ago, can you provide some insights?

Somewhat related:
A discussion on discord: https://discord.com/channels/197033481883222026/1044974379232149535/1048015638502846535
A mod by balthazar to edit unit blueprints to use the feature: https://github.com/The-Balthazar/BrewLAN/tree/master/mods/BrewLAN_Plenae/ExpertCamo

[BlackYps]

And what is going on here?

fa/lua/TerrainTypes.lua

Lines 119 to 127 in fc5368a

	---@alias TerrainEffectType
	---| LayerTerrainEffectType # block type `Layer`
	---| "FXImpact" # block type `ImpactType`
	---| "FXMotionChange" # block type `MotionChangeType`
	---| "FXLayerChange" # block type `LayerChangeType`
	---@alias LayerTerrainEffectType
	---| "FXIdle"
	---| "FXMovement"
	---| "FXOther"

[Hdt80bro]

Which effect table is used is handled here. Those block types are the four ways this function gets called. The FX tables have different key names of the same block type I annotated next to it (So the LayerTerrainEffectType tables all have key names of Layer, the FXImpact table has keys from ImpactType, etc).