Rollup merge of #149837 - scottmcm:wrapping-shift-docs, r=workingjubilee

Update `wrapping_sh[lr]` docs and examples

Inspired by [#general > `Source` link for `core` items is often inscrutable @ 💬][zulip-thread] I wanted to add some more examples of the actual wrapping as well as update the documentation to emphasize that the behaviour is unusual.

In particular, now that `unbounded_sh[lr]` is stable, point people trying to avoid panics to that instead, since it behaves less weirdly.

[zulip-thread]: https://rust-lang.zulipchat.com/#narrow/channel/122651-general/topic/.60Source.60.20link.20for.20.60core.60.20items.20is.20often.20inscrutable/near/562774474

Author: matthiaskrgr