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

Summary by CodeRabbit

• Documentation
  • Updated terrain styling documentation with clarifications about feature capabilities.

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