Auto merge of #111711 - Jules-Bertholet:document-pin-layout, r=thomcc

Document `Pin` memory layout

The fact that `Pin` is `#[repr(transparent)]` technically isn't documented anywhere currently. I don't see any reason why `Pin`'s layout would ever change, so this PR codifies it.

`@rustbot` label +T-libs-api -T-libs +A-docs +A-layout +A-pin

Author: bors