Docs: Correct @return value for _get_block_templates_files().

The function returns `null` if `$template_type` is not `wp_template` or `wp_template_part`.

Follow-up to [52062].

Props dilipbheda, mukesh27.
Fixes #61705.

git-svn-id: https://develop.svn.wordpress.org/trunk@58768 602fd350-edb4-49c9-b593-d223f7449a82

src/wp-includes/block-template-utils.php

@@ -355,7 +355,7 @@ function _get_block_template_file( $template_type, $slug ) {
  *     @type string   $post_type    Post type to get the templates for.
  * }
  *
- * @return array Template
+ * @return array|null Template files on success, null if `$template_type` is not matched.
  */
 function _get_block_templates_files( $template_type, $query = array() ) {
 	if ( 'wp_template' !== $template_type && 'wp_template_part' !== $template_type ) {