Document when cloning occurs

[shepmaster]

Resource uses multiple Clone bounds:

pub trait Resource: Clone {
    type Destination: Clone + Send + Sync + 'static;

For implementors, it would be nice to know a rough estimate of how frequently this happens. Is it per request (a.k.a. "frequently"), per-thread (a.k.a. "not too frequent"), etc.

I think this would be useful whenever a trait requires Clone.