Update NEWS.md for changes to canvas gfx device

NEWS.md

@@ -28,6 +28,8 @@
 
  * The webR HTML `canvas()` graphics device has been moved from the default grDevices package to the webr support package. In addition, it has been reimplemented using the `OffscreenCanvas` API. Users of the Safari web browser will now require at least version 16.4 (the latest stable release at time of writing) to use the canvas graphics device. Users with older browsers not supporting `OffscreenCanvas` will need to use an alternative plotting method, such as bitmap graphics using the `png()` Cairo device.
 
+ * Output messages from the `canvas()` graphics device have been restructured so as to handle more event types in addition to transmitting image bitmap data. For example, an output message is now also emitted when the device creates a new empty plot. WebR apps making use of HTML canvas graphics will need to be updated for the new message structure. See the newly added "Plotting" section in the webR documentation for further details.
+
 ## Bug fixes
 
 * The `REnv` property of a user provided `WebROptions` is now merged with the default value, rather than replacing it. With this a user need not explicitly include the default `R_HOME` value when adding new environment variables.