Enhance manual trailer/video override instructions to support Jellyfin Item IDs and clarify usage

Jellyfin.Plugin.MediaBarEnhanced/Configuration/configPage.html

@@ -123,8 +123,14 @@
                             <div class="fieldDescription" id="customMediaIdsDesc">Enter the IDs of the items you want to show in the slideshow.
                                 You can separate them by new line or comma.
                                 <br><br>
-                                <b>Manual Trailer Override:</b> You can specify a YouTube URL for an item by adding it in
-                                brackets: <br> <code>e.g. ID DESCRIPTION [https://youtu.be/...]</code> or <code>ID [https://youtu.be/...] DESCRIPTION</code>
+                                <b>Manual Trailer/Video Override:</b> You can specify a YouTube URL <b>OR</b> a Jellyfin Item ID (e.g. for a Theme Video) for an item by adding it in
+                                brackets: <br> <code>e.g. ID DESCRIPTION [https://youtu.be/...]</code> or <code>ID [Method] DESCRIPTION</code>.
+                                <br>
+                                Methods:
+                                <ul>
+                                    <li><b>YouTube URL:</b> Play a remote trailer from YouTube.</li>
+                                    <li><b>Jellyfin Item ID (GUID):</b> Play the video of another library item (e.g. a Theme Video or Backdrop Video) using the native player.</li>
+                                </ul>
                                 <br><br>
                                 You can also add a description after the ID using any separator like space, pipe
                                 (|) or dash (-): <br>e.g. <code>ID DESCRIPTION</code> or <code>ID | DESCRIPTION</code>