Distinguish experimental/internal features in rustdoc

[clarfonthey]

Mentioned on Zulip: #t-libs > Add particular remind for internal API

r? rustdoc

[rust-log-analyzer]

The job test-pr-check-2 failed! Check out the build log: (web) (plain enhanced) (plain)

Click to see the possible cause of the failure (guessed by this bot)

[RUSTC-TIMING] askama test:false 0.113
error[E0609]: no field `span` on type `&rustc_attr_ir::Stability`
   --> src/librustdoc/html/render/mod.rs:936:39
    |
936 |         .map(|stab| (stab.level, stab.span, stab.feature))
    |                                       ^^^^ unknown field
    |
    = note: available fields are: `level`, `feature`

For more information about this error, try `rustc --explain E0609`.