Rollup merge of #64529 - taiki-e:docs-pin-as-mut, r=RalfJung
Rollup merge of #64529 - taiki-e:docs-pin-as-mut, r=RalfJung

Add an example to Pin::as_mut

https://github.com/taiki-e/pin-project/issues/89#issuecomment-531701172

r? @RalfJung

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