Rollup merge of #93109 - JakobDegen:arc-docs, r=m-ou-se

Rollup merge of #93109 - JakobDegen:arc-docs, r=m-ou-se

Improve `Arc` and `Rc` documentation

This makes two changes (I can split the PR if necessary, but the changes are pretty small):
 1. A bunch of trait implementations claimed to be zero cost; however, they use the `Arc<T>: From<Box<T>>` impl which is definitely not free, especially for large dynamically sized `T`.
 2.  The code in deferred initialization examples unnecessarily used excessive amounts of `unsafe`. This has been reduced.