No, I mean how did a word meaning castrated cock come to mean a love-letter:seems very odd to me! No success yet with google."Poulet" in French has nearly the same double meaning.Any Shakespeare scholar online at the moment?!
Just realised that your enquiry was about how capon had the double meaning. As you say it is the same meaning in French so perhaps the use of capon by a French character was an "in joke"?