Docs: Improve InstancedMesh page.

docs/api/en/objects/InstancedMesh.html

@@ -54,7 +54,8 @@
 
 		<h3>[property:BufferAttribute instanceMatrix]</h3>
 		<p>
-			Represents the local transformation of all instances. For internal use only.
+			Represents the local transformation of all instances.
+			You have to set its [page:BufferAttribute.needsUpdate needsUpdate] flag to true if you modify instanced data via [page:.setMatrixAt]().
 		</p>
 
 		<h2>Methods</h2>

docs/api/zh/objects/InstancedMesh.html

@@ -54,9 +54,11 @@
 
 		<h3>[property:BufferAttribute instanceMatrix]</h3>
 		<p>
-			Represents the local transformation of all instances. For internal use only.
+			Represents the local transformation of all instances.
+			You have to set its [page:BufferAttribute.needsUpdate needsUpdate] flag to true if you modify instanced data via [page:.setMatrixAt]().
 		</p>
 
+
 		<h2>Methods</h2>
 		<p>See the base [page:Mesh] class for common methods.</p>