Edit rustc_middle::dep_graph module documentation
Edit rustc_middle::dep_graph module documentation

Co-authored-by: Joshua Nelson <d6955d9721560531274cb8f50ff595a9bd39d66f@yottadb.com>
Co-authored-by: Camelid <8177b8a8abced623d23d70ce3ff2dc3fbf53888c@gmail.com>

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