chore: Update config.sample.php to document new ffprobe path option

PR #51712 added new preview_ffprobe_path parameter. This PR updates config.sample.php to document this new optional parameter and the fallback behavior if the option is not set.

Signed-off-by: invario <67800603+invario@users.noreply.github.com>

config/config.sample.php

@@ -1421,6 +1421,15 @@ $CONFIG = [
  */
 'preview_ffmpeg_path' => '/usr/bin/ffmpeg',
 
+/**
+ * custom path for ffprobe binary
+ *
+ * Defaults to ``null`` and falls back to using the same path as ffmpeg.
+ * ffprobe is typically packaged with ffmpeg and is required for
+ * enhanced preview generation for HDR videos.
+ */
+'preview_ffprobe_path' => '/usr/bin/ffprobe',
+
 /**
  * Set the URL of the Imaginary service to send image previews to.
  * Also requires the ``OC\Preview\Imaginary`` provider to be enabled in the