Add single doc comment to librustc/hir/def_id.rs

src/librustc/hir/def_id.rs

@@ -187,6 +187,7 @@ fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
 
 
 impl DefId {
+    /// Make a local `DefId` with the given index.
     pub fn local(index: DefIndex) -> DefId {
         DefId { krate: LOCAL_CRATE, index: index }
     }