Document using download-rustc for working on rustdoc

[jyn514]

This is the easy part of rust-lang/rust#66581. It should go in rustdoc.html.

[jyn514]

I think to close this, we'd just need to add an x setup tools snippet to the start of https://rustc-dev-guide.rust-lang.org/rustdoc.html.

[jieyouxu]

Triage: cheat sheet does have this snippet now.