Clarify that `MultiMesh.set_instance_color()` multiplies vertex colors

This closes https://github.com/godotengine/godot-docs/issues/4273.

(cherry picked from commit 6975bd106ab79cd344a27b27434fb6497bed3564)

doc/classes/MultiMesh.xml

@@ -76,7 +76,7 @@
 			<argument index="1" name="color" type="Color">
 			</argument>
 			<description>
-				Sets the color of a specific instance.
+				Sets the color of a specific instance by [i]multiplying[/i] the mesh's existing vertex colors.
 				For the color to take effect, ensure that [member color_format] is non-[code]null[/code] on the [MultiMesh] and [member SpatialMaterial.vertex_color_use_as_albedo] is [code]true[/code] on the material.
 			</description>
 		</method>