Clarify `preview_max_filesize_image` behavior

Fix https://github.com/nextcloud/server/issues/30220

config/config.sample.php

@@ -1067,9 +1067,10 @@ $CONFIG = [
 'preview_max_y' => 4096,
 
 /**
- * max file size for generating image previews with imagegd (default behavior)
+ * Max file size for generating image previews with imagegd (default behavior).
  * If the image is bigger, it'll try other preview generators, but will most
- * likely show the default mimetype icon. Set to -1 for no limit.
+ * likely either show the default mimetype icon or not display the image at all.
+ * Set to ``-1`` for no limit and try to generate previews on all images sizes.
  *
  * Defaults to ``50`` megabytes
  */