Auto merge of #5346 - derekdreery:package_id_docs, r=matklad
Auto merge of #5346 - derekdreery:package_id_docs, r=matklad

Add docs

Just adds some docs matching what @matklad told me on irc :)

eminence/cargo,nipunn1313/cargo,eminence/cargo,rust-lang/cargo,tsurai/cargo,eminence/cargo,rust-lang/cargo,vojtechkral/cargo,vojtechkral/cargo,nipunn1313/cargo,rustyhorde/cargo,vojtechkral/cargo,eminence/cargo,rust-lang/cargo,rust-lang/cargo,tsurai/cargo,tshepang/cargo,tsurai/cargo,rustyhorde/cargo,rustyhorde/cargo,vojtechkral/cargo,tsurai/cargo,rustyhorde/cargo,tshepang/cargo,tsurai/cargo,tshepang/cargo,nipunn1313/cargo,nipunn1313/cargo,tshepang/cargo