Why give that much detail? If you were given the option to resign at you took it, your resignation is the mark of separation. You can have any number of reasons for resigning, but unless you enumerated them in a letter of resignation, then they exist entirely in your explanation at a later date. Resigned for family issues? Resigned to pursue your rise to become the Holy Dalai Lama (or your life-long fall-back, the King of Norway)? Resigned due to a medical issue? Resigned after receiving a revelation from a potato shaped like Jesus?

Who is going to repudiate that? Your word, your resignation, your explanation. Be as subtle or as grandiose as you wish. It's your story. If the prospective employer visits your former employer by phone or mail, or drops in for a spot of tea and a crumpet (does anybody?), the best they can see is a resignation, to which you'll need to add an explanation. You had cramps that morning, and when the midol wore off, you'd already sent the letter. The chief pilot begged you to stay, but a trout stream in Montana was calling your name, and there were cookies to bake, and Taliban to kill. Spring was in the air. Off you went.

You get the point.