stable rustdoc accepts unstable nightly flags

[matthiaskrgr]

~/.rustup/toolchains/stable-x86_64-unknown-linux-gnu/bin/rustdoc implied-bounds-unconstrained-2.rs "--document-private-items" "--document-hidden-items" -Zunstable-options --edition=2021

will print a bunch of errors but still generate docs it seems.

error: the option `document-hidden-items` is only accepted on the nightly compiler

note: selecting a toolchain with `+toolchain` arguments require a rustup proxy; see <https://rust-lang.github.io/rustup/concepts/index.html>

help: consider switching to a nightly toolchain: `rustup default nightly`

note: for more information about Rust's stability policy, see <https://doc.rust-lang.org/book/appendix-07-nightly-rust.html#unstable-features>

error: the option `Z` is only accepted on the nightly compiler

note: selecting a toolchain with `+toolchain` arguments require a rustup proxy; see <https://rust-lang.github.io/rustup/concepts/index.html>

help: consider switching to a nightly toolchain: `rustup default nightly`

note: for more information about Rust's stability policy, see <https://doc.rust-lang.org/book/appendix-07-nightly-rust.html#unstable-features>