content [nfc]: Add comments linking to CZO, pending API-doc update

The linked discussion is about adding documentation; we can link to
that documentation once it's added.

Author: chrisbobbe

lib/model/content.dart

@@ -1417,9 +1417,14 @@ class _ZulipContentParser {
       thumbnailUrl = src;
     } else if (src.startsWith('/external_content/')
         || src.startsWith('https://uploads.zulipusercontent.net/')) {
+      // This image preview uses camo, which still happens on current servers
+      // (2025-10); discussion:
+      //   https://chat.zulip.org/#narrow/channel/412-api-documentation/topic/documenting.20inline.20images/near/2279235
       srcUrl = src;
       thumbnailUrl = null;
     } else if (href == src)  {
+      // Probably generated by a server before the thumbnailing feature landed:
+      //   https://chat.zulip.org/#narrow/channel/412-api-documentation/topic/documenting.20inline.20images/near/2279234
       srcUrl = src;
       thumbnailUrl = null;
     } else {