Rollup merge of #136351 - Darksonn:coerce-pointee-docs, r=compiler-errors

Add documentation for derive(CoercePointee)

Part of [RFC 3621][rfc] tracked by #123430. This text is heavily based on the guide-level explanation from the RFC.

``@rustbot`` label F-derive_coerce_pointee

[rfc]: https://rust-lang.github.io/rfcs/3621-derive-smart-pointer.html

Author: matthiaskrgr