If you want to add notes onto a specific icon (landing page/traffic source/etc.) all you need to do is:
First, select your desired icon.
Then, click the gear button (bottom left of highlighted icon) to edit the page/icon.
After, the icon's tab will popup on the right side of the canvas and you will be able to edit the icon's:
- Label name
You can add any notes you want for the specific icon.