Add missing newline in docs

Author: sancho20021

Diff for: compiler/rustc_hir/src/hir.rs

@@ -1632,6 +1632,7 @@ pub struct ConstBlock {
 }
 
 /// An expression.
+///
 /// For more details, see the [rust lang reference]
 ///
 /// [rust lang reference]: https://doc.rust-lang.org/reference/expressions.html
@@ -3149,6 +3150,7 @@ impl ItemId {
 /// An item
 ///
 /// The name might be a dummy name in case of anonymous items
+///
 /// For more details, see the [rust lang reference]
 ///
 /// [rust lang reference]: https://doc.rust-lang.org/reference/items.html