You could do it as a comment (Insert, Comment), but that would show up either at the side or bottom of the document, depending on your settings.
You could try doing it as a hyperlink that links to the current document (i.e. goes nowhere), but this would allow you to enter some text as a Screen Tip (in the hyperlink dialog box). The screen tip text would show up when you hover over the hyperlink. (Screen grab below is from a Mac, but would be similar on PC.)
Cheers
Steve_D

Attached Images

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