Improve inline documentation for `wpdb::has_cap()`.

Adds a changelog entry for the addition of the 'utf8mb4' feature, accepted values for `$db_cap`, and a return description.

Props kpdesign.
See #21212.

Built from https://develop.svn.wordpress.org/trunk@30768


git-svn-id: http://core.svn.wordpress.org/trunk@30758 1a063a9b-81f0-0310-95a4-ce76da25c4cd

wp-includes/version.php

@@ -4,7 +4,7 @@
  *
  * @global string $wp_version
  */
-$wp_version = '4.1-beta2-30767';
+$wp_version = '4.1-beta2-30768';
 
 /**
  * Holds the WordPress DB revision, increments when changes are made to the WordPress DB schema.

wp-includes/wp-db.php

@@ -2752,10 +2752,14 @@ class wpdb {
 	 * Determine if a database supports a particular feature.
 	 *
 	 * @since 2.7.0
+	 * @since 4.1.0 Support was added for the 'utf8mb4' feature.
+	 *
 	 * @see wpdb::db_version()
 	 *
-	 * @param string $db_cap The feature to check for.
-	 * @return bool
+	 * @param string $db_cap The feature to check for. Accepts 'collation',
+	 *                       'group_concat', 'subqueries', 'set_charset',
+	 *                       or 'utf8mb4'.
+	 * @return bool Whether the database feature is supported, false otherwise.
 	 */
 	public function has_cap( $db_cap ) {
 		$version = $this->db_version();