Including unstable lints in the lint group produces unintuitive behavior
on stable (see #106289). Meanwhile, if we only included unstable lints
on nightly and not on stable, we could end up with confusing bugs that
were hard to compare across versions of Rust that lacked code changes.
I think that only including stable lints in `rustdoc::all`, no matter
the release channel, is the most intuitive option. Users can then
control unstable lints individually, which is reasonable since they have
to enable the feature gates individually anyway.
