Update comment.

Author: mbostock

src/fileAttachment.js

@@ -49,7 +49,7 @@ export function NoFileAttachments(name) {
 
 export default function FileAttachments(resolve) {
   return name => {
-    const url = resolve(name += ""); // Returns a Promise, or null.
+    const url = resolve(name += ""); // Returns a Promise, string, or null.
     if (url == null) throw new Error(`File not found: ${name}`);
     return new FileAttachment(url, name);
   };