Clarify jump target cache behavior

[pdonahue-ventana]

I've discussed the jump target cache behavior with multiple people and there's some confusion about whether the cache index comes from the PC of the jump or the PC of the target. Can you change the jump target cache section from "its index" to "the index of the jump target"? (The only "it" in that sentence is "uninferable jump target" so it's already clear to me, but it could apparently be clearer.)