Rollup merge of #138080 - moxian:bootstrap-breadcrumb, r=jieyouxu

Leave a breadcrumb towards bootstrap config documentation in `bootstrap.toml`

I was curious as to the possible bootstrap options I can put in config.toml, but had some trouble figuring it out. There is no obvious documentation in `config.toml` (obviously), the documentation in src/bootstrap/defaults is *nice*, but also rather sparse, by design.
I had to dive into the parsing code, and stuble upon [a very helpful doc comment](https://github.com/rust-lang/rust/blob/30f168ef811aec63124eac677e14699baa9395bd/src/bootstrap/src/core/config/config.rs#L181) there to realize that `config.example.toml` *exists*, and that it does, indeed, answer all of my questions.

So I figured it might be worth making this journey a bit easier for future contributors and add mention the `config.example.toml` directly in `config.toml`.

Now, since #137081 is in-flight which would rename `config(.example)?.toml` to `bootstrap(.example)?.toml`, I figure it's better to wait until that one lands, and submit the "new", "correct" filename upfront, instead of landing `config.toml` now and updating it to `boostrap.toml` later.

`@rustbot` blocked #137081

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