Docs: Add a missing notation for the `$gmt_time` parameter in the DocBlock for `spawn_cron()`.

See #32246.


git-svn-id: https://develop.svn.wordpress.org/trunk@36022 602fd350-edb4-49c9-b593-d223f7449a82

src/wp-includes/cron.php

@@ -232,9 +232,11 @@ function wp_next_scheduled( $hook, $args = array() ) {
 }
 
 /**
- * Send request to run cron through HTTP request that doesn't halt page loading.
+ * Sends a request to run cron through HTTP request that doesn't halt page loading.
  *
  * @since 2.1.0
+ *
+ * @param int $gmt_time Optional. Unix timestamp. Default 0 (current time is used).
  */
 function spawn_cron( $gmt_time = 0 ) {
 	if ( ! $gmt_time )