Right-click the activity and choose Annotations > Add annotation. A little yellow note card will appear, where you can note some important things. It disappears when you click on the flowchart, but its icon will remain on the activity.
You can read the note hovering the mouse over the annotations icon.
Click the pin in upper right corner of the yellow card to make the note permanently show up on the activity's handler and click the pin once again to hide it.
With the commands show all annotations/hide all annotations you can manage all the notes in the current scenario at once.