Add comment and clean up expand_annotatable

src/libsyntax/ext/base.rs

@@ -942,6 +942,8 @@ impl SyntaxEnv {
     }
 
     pub fn is_crate_root(&mut self) -> bool {
+        // The first frame is pushed in `SyntaxEnv::new()` and the second frame is
+        // pushed when folding the crate root pseudo-module (c.f. noop_fold_crate).
         self.chain.len() == 2
     }
 }