Add changelog entries to the `post_gallery` and `post_playlist` hook docs for the `$instance` variable that was added in [31304].

Props ninnypants for the initial patch.
Fixes #31151.


git-svn-id: https://develop.svn.wordpress.org/trunk@31309 602fd350-edb4-49c9-b593-d223f7449a82

src/wp-includes/media.php

@@ -938,6 +938,7 @@ function gallery_shortcode( $attr ) {
 	 * the default gallery template.
 	 *
 	 * @since 2.5.0
+	 * @since 4.2.0 The `$instance` parameter was added.
 	 *
 	 * @see gallery_shortcode()
 	 *
@@ -1214,6 +1215,7 @@ function wp_playlist_shortcode( $attr ) {
 	 * of the default playlist output, returning the passed value instead.
 	 *
 	 * @since 3.9.0
+	 * @since 4.2.0 The `$instance` parameter was added.
 	 *
 	 * @param string $output   Playlist output. Default empty.
 	 * @param array  $attr     An array of shortcode attributes.