Document newtype_index attributes.

compiler/rustc_index_macros/src/lib.rs

@@ -18,8 +18,19 @@ mod newtype;
 ///   to create/return a value.
 ///
 /// Internally, the index uses a u32, so the index must not exceed
-/// `u32::MAX`. You can also customize things like the `Debug` impl,
-/// what traits are derived, and so forth via the macro.
+/// `u32::MAX`.
+///
+/// The impls provided by default are Clone, Copy, PartialEq, Eq, and Hash.
+///
+/// Accepted attributes for customization:
+/// - #[derive(HashStable_Generic)]/#[derive(HashStable)]: derives
+///   `HashStable`, as normal.
+/// - #[encodable]: derives `Encodable`/`Decodable`.
+/// - #[orderable]: derives `PartialOrd`/`Ord`, plus step-related methods.
+/// - #[debug_format = "Foo({})"]: derives `Debug` with particular output.
+/// - #[max = 0xFFFF_FFFD]: specifies the max value, which allows niche
+///   optimizations. The default max value is 0xFFFF_FF00.
+/// - #[gate_rustc_only]: makes parts of the generated code nightly-only.
 #[proc_macro]
 #[cfg_attr(
     feature = "nightly",