More config.toml.example cleanups

- Link to more documentation
- Move `changelog-seen` into the "Global Settings" section
- Update incorrect comments on `llvm.link-shared` and
  `rust.debug-assertions`
- Use the correct default in the commented-out example more often
- Clarify that `docs` and `compiler-docs` only control the default,
  they're not a hard-off switch.
- Document `-vvv` and `local-rebuild`
- Minor improvements to doc-comments in config.toml.example

This also sets `download-rustc = false`; that was already the default,
but it will be helpful in case the default changes
(https://jyn.dev/2023/01/12/Bootstrapping-Rust-in-2023.html).