The captured values are moved when the closure is called, not when it's constructed#118

_WARNING_ I'm a n00b to Rust, so this is likely incorrect! Please review carefully, and apologies if this is incorrect 😀. While closures can capture some environment by moving, it's not entirely clear to me _when_ this happens. I guess it happens when the closure is called, not when the closure is created. Hence, I'm proposing to add a few words. I don't think this is made entirely clear elsewhere in the document.