Rewrite the section on passing flags to subcommands
Rewrite the section on passing flags to subcommands

- Move the reference for directories and actions to the very end; it's the most rare to need to know
- Add `RUSTDOCFLAGS*`, `CARGOFLAGS*`, `-vvv`, and `--test-args`
- Remove the incorrect `--on-fail` command

rust-lang/rustc-dev-guide,rust-lang/rustc-dev-guide,rust-lang/rustc-dev-guide,rust-lang/rustc-dev-guide