Docs: Add documentation for the `wp_maybe_auto_update` action.

Props MikeGillihan.
Fixes #43301.



git-svn-id: https://develop.svn.wordpress.org/trunk@44609 602fd350-edb4-49c9-b593-d223f7449a82

src/wp-includes/update.php

@@ -232,6 +232,11 @@ function wp_version_check( $extra_stats = array(), $force_check = false ) {
 
 	// Trigger background updates if running non-interactively, and we weren't called from the update handler.
 	if ( $doing_cron && ! doing_action( 'wp_maybe_auto_update' ) ) {
+		/**
+		 * Fires during wp_cron, starting the auto update process.
+		 *
+		 * @since 3.9.0
+		 */
 		do_action( 'wp_maybe_auto_update' );
 	}
 }