Clarify how to convert PrimitiveMesh to ArrayMesh.

It took me a bit to figure this out, as I was initially doing something
more complicated like this before I realized I just had to pass
get_mesh_arrays directly to add_surface_from_arrays.

```
var arr_mesh = ArrayMesh.new()
var arrays = []
arrays.resize(ArrayMesh.ARRAY_MAX)
arrays[ArrayMesh.ARRAY_VERTEX] = c.get_mesh_arrays()
arr_mesh.add_surface_from_arrays(Mesh.PRIMITIVE_TRIANGLES, arrays)
```

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