Improve comments

src/librustc/hir/intravisit.rs

@@ -203,7 +203,7 @@ pub trait Visitor<'v>: Sized {
 
     /// Invoked to visit the body of a function, method or closure. Like
     /// visit_nested_item, does nothing by default unless you override
-    /// `nested_visit_map` to return other htan `None`, in which case it will walk
+    /// `nested_visit_map` to return other than `None`, in which case it will walk
     /// the body.
     fn visit_nested_body(&mut self, id: BodyId) {
         let opt_body = self.nested_visit_map().intra().map(|map| map.body(id));

src/librustc/hir/mod.rs

@@ -2512,7 +2512,7 @@ pub enum ItemKind {
     Fn(P<FnDecl>, FnHeader, Generics, BodyId),
     /// A module.
     Mod(Mod),
-    /// An external module.
+    /// An external module, e.g. `extern { .. }`.
     ForeignMod(ForeignMod),
     /// Module-level inline assembly (from `global_asm!`).
     GlobalAsm(P<GlobalAsm>),