Two minor model thing notes (#257)

* Add note regarding global vertex IDs in b3d weights

* Add warning regarding buggy Blockbench versions

Author: appgurueu

content/for-creators/models/b3d-spec.md

@@ -167,6 +167,13 @@ float weight                ; how much the vertex is affected
 The `BONE` chunk describes a bone. Weights are applied to the mesh described in the enclosing `ANIM` -
 in 99% of cases, this will simply be the `MESH` contained in the root `NODE` chunk.
 
+{{< notice note >}}
+There may be multiple meshes containing multiple sets of vertices
+("mesh buffers" in Irrlicht parlance),
+yet there is only a single `vertex_id` for each weight.
+This ID is global and refers to the order vertices appear in the file (c.f. `CB3DMeshFileLoader.(cpp|h)` in Irrlicht).
+{{< /notice >}}
+
 ### `KEYS`
 
 ```

content/for-creators/models/using-blockbench.md

@@ -76,6 +76,10 @@ While Blockbench uses an internal block size of 16, Luanti expects a different s
 
 ### glTF
 
+{{< notice warning >}}
+Blockbench versions prior to 4.7.0 suffer from [a bug](https://github.com/JannisX11/blockbench/issues/1743) which flips texture coordinates along the Y axis when exporting glTF models. Using a recent Blockbench version is **strongly recommended**. (Alternatively you can also fix the UV coordinates in Blender, or flip the textures as a workaround.)
+{{< /notice >}}
+
 Luanti expects `gltf` files to use a node size of 10,
 which requires Blockbench to scale down its internal model by a factor of `1.6 = 16 / 10`.