You add a layer to the comp. You highlight the layer. You hit the asterisk key on the numeric keypad. This creates a marker. If you double-click on the marker, you can add text to annotate the marker. I'm not sure, but you may be limited to 64 characters. You'd have to check for sure.
When you think about it, this is a pretty handy way to do it. You can leave notes to yourself or others on what happens at a particular point in time.
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