Document the update detection mechanism of VersionedCanvas::update

src/versioned_canvas.rs

@@ -45,6 +45,10 @@ impl VersionedCanvas {
         }
     }
 
+    // TODO: Right now the update function internally checks
+    // whether the canvas has been changed by leveraging the
+    // Copy on Write semantics of im::Vector.
+    // Is this a good solution? Does this work correctly? THINK ABOUT THIS
     pub fn update(&mut self, update_fn: impl FnOnce(&mut Canvas)) {
         // This is a linear history,
         // so we first check if there are newer versions, if so