docs: add custom model variant examples to block-propertie

doc/en/block-properties.md

@@ -98,6 +98,32 @@ Properties available for variance:
 
 Variants are managed via `block.set_variant(x, y, z, index)`.
 
+### Custom model variants (geometry switching)
+
+You can use different custom models for different variants. Provide a separate `model-name` for each variant that needs different geometry. The renderer caches geometry per (block id, variant).
+
+The base model (specified in root) becomes variant 0. The variants array maps to indices 1+.
+
+Example (default + two custom variants):
+```json
+{
+    "model": "custom",
+    "model-name": "stairs_middle",
+    "state-based": {
+        "bits": 4,
+        "variants": [
+            { "model": "custom", "model-name": "stairs_left" },
+            { "model": "custom", "model-name": "stairs_right" }
+        ]
+    }
+}
+```
+
+In this example:
+- Variant 0 = `stairs_middle` (from root)
+- Variant 1 = `stairs_left` (from variants[0])
+- Variant 2 = `stairs_right` (from variants[1])
+
 ## Lighting
 
 ### *emission*