Auto merge of #6815 - ehuss:doc-downloads, r=Eh2406
Auto merge of #6815 - ehuss:doc-downloads, r=Eh2406

Add some docs for `Downloads`.

Rather than fight with rustfmt which didn't like the inline comment, I decided to just add some more elaborate docs.

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