Docs: Add missing documentation for the first parameter of the big_image_size_threshold filter.

Props dkarfa, desrosj, earnjam.
Fixes #48214.

git-svn-id: https://develop.svn.wordpress.org/trunk@46398 602fd350-edb4-49c9-b593-d223f7449a82

src/wp-admin/includes/image.php

@@ -235,6 +235,7 @@ function wp_create_image_subsizes( $file, $attachment_id ) {
 	 *
 	 * @since 5.3.0
 	 *
+	 * @param int    $threshold     The threshold value in pixels. Default 2560.
 	 * @param array  $imagesize     Indexed array of the image width and height (in that order).
 	 * @param string $file          Full path to the uploaded image file.
 	 * @param int    $attachment_id Attachment post ID.