Rollup merge of #44773 - GuillaumeGomez:arc-docs, r=@QuietMisdreavus
Rollup merge of #44773 - GuillaumeGomez:arc-docs, r=@QuietMisdreavus

Add missing links for Arc

r? @rust-lang/docs

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