Good description, more than enough examples; it just needs a few more categories. Let's find some and get this out into the main namespace.
Talk:Marginal Note
It's got the categories now, but it also occurs to me that having some snarky text in a sidemenu commenting on the main article would be right on target. Let me come up one and I'll launch it.
Wasn't there an example in one of the Harry Potter books (and its corresponding movie) where Harry gets Snape's (I think it was Snape's) old textbook with his notes in it? I don't know the series well enough to write such an entry.
That example has been added to the page in the last couple hours, probably in response to your query.
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