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

0 files changed, 0 insertions, 0 deletions