Docs: Clarify the usage of `null` for `auto_update_{$type}` filter.

The value is internally used to detect whether nothing has hooked into this filter.

Props audrasjb, pbiron, johnbillion, SergeyBiryukov.
Merges [48909] to the 5.5 branch.
Fixes #50848.
Built from https://develop.svn.wordpress.org/branches/5.5@48916


git-svn-id: http://core.svn.wordpress.org/branches/5.5@48678 1a063a9b-81f0-0310-95a4-ce76da25c4cd

wp-admin/includes/class-wp-automatic-updater.php

@@ -195,8 +195,10 @@ class WP_Automatic_Updater {
 		 * adjust core updates.
 		 *
 		 * @since 3.7.0
+		 * @since 5.5.0 The `$update` parameter accepts the value of null.
 		 *
-		 * @param bool   $update Whether to update.
+		 * @param bool|null $update Whether to update. The value of null is internally used
+		 *                          to detect whether nothing has hooked into this filter.
 		 * @param object    $item   The update offer.
 		 */
 		$update = apply_filters( "auto_update_{$type}", $update, $item );

wp-includes/version.php

@@ -13,7 +13,7 @@
  *
  * @global string $wp_version
  */
-$wp_version = '5.5.1-RC1-48915';
+$wp_version = '5.5.1-RC1-48916';
 
 /**
  * Holds the WordPress DB revision, increments when changes are made to the WordPress DB schema.