Clarify the compositor a bit

Manishearth · toji · commit bca723297c54 · 2019-08-14T16:19:45.000-07:00
diff --git a/index.bs b/index.bs
@@ -900,7 +900,9 @@ Applications which use {{XRSessionMode/"inline"}} sessions for rendering to the
 The XR Compositor {#compositor}
 -----------------
 
-The user agent MUST maintain an <dfn>XR Compositor</dfn> which handles presentation to the [=XRSession/XR device=] and frame timing. The compositor MUST use an independent rendering context whose state is isolated from that of any WebGL contexts used as {{XRWebGLLayer}} sources to prevent the page from corrupting the compositor state or reading back content from other pages. the compositor MUST also run in separate thread or processes to decouple performance of the page from the ability to present new imagery to the user at the appropriate framerate.
+The user agent MUST maintain an <dfn>XR Compositor</dfn> which handles presentation to the [=XRSession/XR device=] and frame timing. The compositor MUST use an independent rendering context whose state is isolated from that of any WebGL contexts used as {{XRWebGLLayer}} sources. The compositor MUST prevent the page from corrupting the compositor state or reading back content from other pages or applications. The compositor MUST also run in separate thread or processes to decouple performance of the page from the ability to present new imagery to the user at the appropriate framerate. The compositor MAY composite additional device or user agent UI over rendered content, like device menus.
+
+NOTE: Future extensions to this spec may utilize the compositor to composite multiple layers coming from the same page as well.
 
 Frame Loop {#frame}
 ==========
