Add inline comments explaining subtitle field

Added comments to clarify that Atom feeds use <subtitle> for feed description.

Co-authored-by: openhands <openhands@all-hands.dev>

Author: openhands-agent

index.d.ts

@@ -59,7 +59,7 @@ declare namespace Parser {
     items: (U & Item)[];
     feedUrl?: string;
     description?: string;
-    subtitle?: string;
+    subtitle?: string; // Atom feeds use <subtitle> for feed description
     itunes?: {
       [key: string]: any;
       image?: string;

lib/fields.js

@@ -9,7 +9,7 @@ fields.feed = [
   ['dc:type', 'type'],
   'title',
   'description',
-  'subtitle',
+  'subtitle', // Atom feeds use <subtitle> for feed description
   'author',
   'pubDate',
   'webMaster',