You can add a note, assigned to a certain stat, in an automatic popup, which opens when you hover over the stat. After that you can find a written note in this automatic popup:
Also, you can write simple notes:
Create a new popup in Popup Editor.
Insert Note Groups and rename them
Assign this popup to a stat in the HUD
After you write some note you will see it at the note icon
If you don't have a note icon in your HUD, you can add it in HUD Editor:
