In truth, it's very difficult to add meaningful annotations after you have finalized the wireframe. The best practice is to annotate as you go and finalize your notes once you have finished.

Annotations are there to explain things that are impossible to illustrate with the wireframe, such as design rationale, scenarios, edge cases, or interactions.^[1] What happens when users interact with these elements? Is this banner shown to all users or only those who are redirected from certain pages? Annotate it. You've succeeded if developers or clients can read your notes and understand not only what that element does but also why.