Explain the terrain modes

**Your Godot version:** 4

**Issue description:**
I went to the docs to try to figure out what the terrain sets modes mean.
Instead of an explanation, there's this:

> Terrains are grouped into terrain sets. Each terrain set is assigned a mode from Match Corners and Sides, Match Corners and Match sides. They define how terrains are matched to each other in a terrain set.
>
> Note: The above modes correspond to the previous bitmask modes autotiles used in Godot 3.x: 2×2, 3×3 or 3×3 minimal. This is also similar to what the [Tiled](https://www.mapeditor.org/) editor features.

this is completely useless to someone who doesn't already know what autotiling did in godot 3, or how tiled works. there should be an explanation for what each mode looks like, when to use each, with pictures.

Or you could at least link to the actual documentation page of tiled that explains it, instead of just their homepage: https://doc.mapeditor.org/en/stable/manual/terrain/

But honestly I still dont understand their explanation.

**URL to the documentation page (if already existing):**
https://docs.godotengine.org/en/stable/tutorials/2d/using_tilesets.html#creating-terrain-sets-autotiling