script: Document need for always sending an image update to compositor (#39210)

I hit this many times while working on #38717

Testing: Not needed because we just update the docs

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Signed-off-by: Sam <16504129+sagudev@users.noreply.github.com>
Co-authored-by: Martin Robinson <mrobinson@igalia.com>

components/script/dom/canvas/canvas_context.rs

@@ -108,6 +108,11 @@ pub(crate) trait CanvasContext {
 
     /// Request that the [`CanvasContext`] update the rendering of its contents,
     /// returning `true` if new image was produced.
+    ///
+    /// Note: If this function returns `true`, script will wait for all images to be updated
+    /// before updating the rendering again. Be sure that image updates are always sent
+    /// even in the failure case by sending transparent black image or return `false` in the
+    /// case of failure.
     fn update_rendering(&self, _canvas_epoch: Epoch) -> bool {
         false
     }