Add inline comments explaining subtitle field additions

Added comments to clarify that the subtitle fields in fields.feed and
fields.item support standard RSS/Atom subtitle elements, which are
distinct from the itunes:subtitle fields in podcastFeed and podcastItem.

Author: openhands-agent

lib/fields.js

@@ -8,7 +8,7 @@ fields.feed = [
   ['dc:title', 'title'],
   ['dc:type', 'type'],
   'title',
-  'subtitle',
+  'subtitle', // Added to support standard RSS/Atom subtitle elements (distinct from itunes:subtitle in podcastFeed)
   'description',
   'author',
   'pubDate',
@@ -36,7 +36,7 @@ fields.item = [
   ['dc:source', 'source'],
   ['dc:title', 'title'],
   'title',
-  'subtitle',
+  'subtitle', // Added to support standard RSS/Atom subtitle elements (distinct from itunes:subtitle in podcastItem)
   'link',
   'pubDate',
   'author',