Rollup merge of #141215 - xizheyin:issue-141138, r=workingjubilee

std: clarify Clone trait documentation about duplication semantics

Closes rust-lang/rust#141138

The change explicitly explains that cloning behavior varies by type and clarifies that smart pointers (`Arc`, `Rc`) share the same underlying data. I've also added an example of cloning to Arc.

Author: jhpratt