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Rollup merge of #139885 - jyn514:doc-rustc-bootstrap, r=Nadrieril
document RUSTC_BOOTSTRAP, RUSTC_OVERRIDE_VERSION_STRING, and -Z allow-features in the unstable book
[rendered](https://github.com/jyn514/rust/blob/doc-rustc-bootstrap/src/doc/unstable-book/src/compiler-flags/rustc-bootstrap.md)
mcp: https://github.com/rust-lang/compiler-team/issues/863
