It’s missing a fifth symbol: a set of three wavy lines next to each other.
Ah, I see my memory was wrong. That would have been true if the triangle was instead a five-pointed star, and the X was a plus sign.
I think the next symbols would be R1, R2, L1, and L2 ;-) (Those are the symbols on the buttons of a PlayStation controller.)
Incidentally, the notes in early editions of The Rise and Fall of the Roman Empire, the book in which the invention of the footnote is said to have taken place, actually placed the notes in the margins.
The reason this was abandoned for footnotes is obvious - it wasn't because footnotes were easier on the reader, but because they took less space.
Sidenotes are better for the reader, but they need to be concise, and sparse (which, by the way, would be good advice for any kind of note ;-) thus creating ample whitespace where they're not present.
Unfortunately, in print whitespace in the form of wide margins come (literally) with a cost.
But, had we had more of them, maybe we wouldn't had to wait 358 years for a proof of Fermat's last theorem... ;-)