Docs: Add a @since note for get_post_permalink() returning false on failure.

Follow-up to [53733].

See #45329.

git-svn-id: https://develop.svn.wordpress.org/trunk@53734 602fd350-edb4-49c9-b593-d223f7449a82

src/wp-includes/link-template.php

@@ -308,6 +308,7 @@ function get_permalink( $post = 0, $leavename = false ) {
  * Retrieves the permalink for a post of a custom post type.
  *
  * @since 3.0.0
+ * @since 6.1.0 Returns false if the post does not exist.
  *
  * @global WP_Rewrite $wp_rewrite WordPress rewrite component.
  *