docs: add detail for active date of thumbnails (#487)

Author: 0xcadams

developers/guides/thumbnails-vod.mdx

@@ -16,6 +16,12 @@ Follow our previous guide on
 [uploading an asset](/developers/guides/upload-video-asset) and get the
 `playbackId`.
 
+<Info>
+  Any assets uploaded after November 21st, 2023 will have thumbnails generated
+  for them. If the asset was uploaded before then, it will need to be
+  re-imported to create thumbnails.
+</Info>
+
 ## Fetch playback info
 
 Once the processing has completed for your asset, fetch the playback info using