clarify the example in SkinnedMesh docs (#23423)

docs/api/en/objects/SkinnedMesh.html

@@ -38,7 +38,8 @@
 		<code>
 		const geometry = new THREE.CylinderGeometry( 5, 5, 5, 5, 15, 5, 30 );
 
-		// create the skin indices and skin weights
+		// create the skin indices and skin weights manually
+		// (typically a loader would read this data from a 3D model for you)
 
 		const position = geometry.attributes.position;