Make Arc cloning mechanics clearer in module docs
Add some more wording to module documentation regarding how
`Arc::clone()` works, as some users have assumed cloning Arc's
to work via dereferencing to inner value as follows:
use std::sync::Arc;
let myarc = Arc::new(1);
let myarcref = myarc.clone();
assert!(1 == myarcref);
Instead of the actual mechanic of referencing the existing
Arc value:
use std::sync::Arg;
let myarc = Arc::new(1);
let myarcref = myarc.clone();
assert!(myarcref == &myarc); // not sure if assert could assert this in the real world
fcd76b4d3c