Auto merge of #5040 - daboross:patch-1, r=alexcrichton

Remove cargo-only downloads from installation docs

This also copies extra instructions for installing rust from https://doc.rust-lang.org/book/first-edition/getting-started.html#installing-rust-1.

I wasn't able to test the changes locally, but they're fairly minimal, so I trust they will render alright.

Fixes #5027.