Database: Add documentation for the allowed `utf8mb4_520` value when detecting features with `wpdb::has_cap()`.

Props rcutmore

Fixes #41044

Built from https://develop.svn.wordpress.org/trunk@40906


git-svn-id: http://core.svn.wordpress.org/trunk@40756 1a063a9b-81f0-0310-95a4-ce76da25c4cd

wp-includes/version.php

@@ -4,7 +4,7 @@
  *
  * @global string $wp_version
  */
-$wp_version = '4.9-alpha-40905';
+$wp_version = '4.9-alpha-40906';
 
 /**
  * Holds the WordPress DB revision, increments when changes are made to the WordPress DB schema.

wp-includes/wp-db.php

@@ -3268,7 +3268,7 @@ class wpdb {
 	 *
 	 * @param string $db_cap The feature to check for. Accepts 'collation',
 	 *                       'group_concat', 'subqueries', 'set_charset',
-	 *                       or 'utf8mb4'.
+	 *                       'utf8mb4', or 'utf8mb4_520'.
 	 * @return int|false Whether the database feature is supported, false otherwise.
 	 */
 	public function has_cap( $db_cap ) {