Update subtree sync docs for changes in rustc-dev-guide
Companion PR to https://github.com/rust-lang/rustc-dev-guide/pull/1653. That still keeps a `contributing.html` with an "External Contributions" section, so hopefully it won't be too disruptive.
changelog: none
- Move doc about defining remotes to the front and use the defined
remotes in the further documentation
- Don't recommend pushing to the remote repo directly
- Add some clarifying comments