Docs: Add return value description for `register_theme_directory()`.

Props stevenlinx.
Fixes #50665.
Built from https://develop.svn.wordpress.org/trunk@48491


git-svn-id: http://core.svn.wordpress.org/trunk@48253 1a063a9b-81f0-0310-95a4-ce76da25c4cd

wp-includes/theme.php

@@ -397,7 +397,8 @@ function get_theme_roots() {
  *
  * @param string $directory Either the full filesystem path to a theme folder
  *                          or a folder within WP_CONTENT_DIR.
- * @return bool
+ * @return bool True if successfully registered a directory that contains themes,
+ *              false if the directory does not exist.
  */
 function register_theme_directory( $directory ) {
 	global $wp_theme_directories;

wp-includes/version.php

@@ -13,7 +13,7 @@
  *
  * @global string $wp_version
  */
-$wp_version = '5.5-beta2-48490';
+$wp_version = '5.5-beta2-48491';
 
 /**
  * Holds the WordPress DB revision, increments when changes are made to the WordPress DB schema.