docs: clarify MaskAnnotator mask requirements

docs/detection/annotators.md

@@ -194,6 +194,16 @@ Annotators accept detections and apply box or mask visualizations to the detecti
         )
         ```
 
+        !!! note
+
+            `MaskAnnotator` expects `detections.mask` to contain instance segmentation
+            masks aligned to the image passed to `annotate`. For dense masks, provide a
+            boolean array of shape `(N, H, W)` where `(H, W)` matches the image height
+            and width (it also accepts `sv.CompactMask`). If your model returns
+            framework-specific results, convert them to `sv.Detections` first, for
+            example with `sv.Detections.from_ultralytics(...)` or
+            `sv.Detections.from_inference(...)`.
+
         <div class="result" markdown>
 
         ![mask-annotator-example](https://media.roboflow.com/supervision-annotator-examples/mask-annotator-example-purple.png){ align=center width="800" }