Explain how to "ask for another round of review" via rustbot labels

[RalfJung]

New contributors currently have no good way of knowing this magic incantation

@rustbot label -S-waiting-on-author +S-waiting-on-review

It should probably be mentioned somewhere on https://rustc-dev-guide.rust-lang.org/contributing.html.

See rust-lang/rust#82703 for a recent case where this came up (and I've certainly seen other instances of this before).

[iago-lito]

Well, now I have found it there, pointing there. The problem was that I was so busy figuring out the How to Build and Run the Compiler and the compiler testing framework back then that I did not make my way down to this section.

But now that I am mostly done with implementation, maybe I should've simply come back to this book earlier?

[RalfJung]

Ah, that also shows a good place to add the exact command. :) Opened #1124.