Ash doesn't implement `Drop` intentionally, to not be too opinionated
about holding (heap) references to their parent objects
(`Device`->`Instance`->`Entry`) and ensuring they are destroyed in the
right order. As such, reword the `create` documentation for `Instance`
and `Device` to mention their respective `destroy_*` function instead of
referring to them as being "droppable".
Note that `Entry` is droppable as it does not have a Vulkan `destroy`
function _and_ the dynamically loaded library (behind the "loaded"
feature) is kept alive only for the lifetime of `Entry`.
1e7ee6762f