[impeller] adds experimental canvas docstring (#52710)

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Author: gaaclarke

impeller/aiks/experimental_canvas.h

@@ -18,6 +18,13 @@
 
 namespace impeller {
 
+/// This Canvas attempts to translate from display lists to draw calls directly.
+///
+/// It's not fully implemented yet but if successful it will be replacing the
+/// aiks Canvas functionality.
+///
+/// See also:
+///   - go/impeller-canvas-efficiency
 class ExperimentalCanvas : public Canvas {
  public:
   ExperimentalCanvas(ContentContext& renderer,