Rollup merge of #44972 - durka:patch-44, r=arielb1

fix ItemKind::DefaultImpl doc comment

Upgrade comment to doc comment.

...Is this actually used? If so, why does the `Impl` variant right below have a `Defaultness`?

src/libsyntax/ast.rs

@@ -1927,7 +1927,7 @@ pub enum ItemKind {
     ///
     /// E.g. `trait Foo { .. }` or `trait Foo<T> { .. }`
     Trait(Unsafety, Generics, TyParamBounds, Vec<TraitItem>),
-    // Default trait implementation.
+    /// Auto trait implementation.
     ///
     /// E.g. `impl Trait for .. {}` or `impl<T> Trait<T> for .. {}`
     DefaultImpl(Unsafety, TraitRef),