Leland Ryken notes:
Hamlet's friend Horatio quips, "I knew you must be edified by the margent ere you had done." A colloquial version of that would be, "I knew you would need an explanatory marginal note to figure that one out." "Edified by the margent:" this is an allusion to a statement in the preface to the Geneva Bible, where the editors tell the readers that for "good purpose and edification" they "have in the margent noted" certain things. "Edification ... in the margent;" "edified by the margent." (http://www.reformation21.org/articles/shakespeare-and-the-geneva-bible.php)
HT: JT
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