Rustdoc is being documented without --document-private-items on nightly-rustc

[jyn514]

Compare https://doc.rust-lang.org/beta/nightly-rustc/rustdoc/ to https://doc.rust-lang.org/nightly/nightly-rustc/rustdoc/.

This is almost certainly caused by #86737.

[Stupremee]

@rustbot claim