Rollup merge of #53104 - nivkner:unpin_doc, r=RalfJung
Rollup merge of #53104 - nivkner:unpin_doc, r=RalfJung

expand the documentation on the `Unpin` trait

provides an overview of the Pin API which the trait is for,
and show how it can be used in making self referencial structs

part of #49150

aidancully/rust,aidancully/rust,aidancully/rust,GBGamer/rust,aidancully/rust,graydon/rust,graydon/rust,graydon/rust,graydon/rust,aidancully/rust,GBGamer/rust,GBGamer/rust,GBGamer/rust,aidancully/rust,GBGamer/rust,GBGamer/rust,graydon/rust,graydon/rust,GBGamer/rust,GBGamer/rust