Clairify docs for AttributeKind::DocComment

Makes it clear that this represents `#[doc = "content"]`, but not
`#[doc(hidden)]` (or other uses of the `#[doc(...)]` attribute).

Author: aDotInTheVoid

compiler/rustc_hir/src/attrs/data_structures.rs

@@ -516,7 +516,7 @@ pub enum AttributeKind {
     /// Represents `#[rustc_do_not_implement_via_object]`.
     DoNotImplementViaObject(Span),
 
-    /// Represents [`#[doc]`](https://doc.rust-lang.org/stable/rustdoc/write-documentation/the-doc-attribute.html).
+    /// Represents [`#[doc = "..."]`](https://doc.rust-lang.org/stable/rustdoc/write-documentation/the-doc-attribute.html).
     DocComment { style: AttrStyle, kind: CommentKind, span: Span, comment: Symbol },
 
     /// Represents `#[rustc_dummy]`.