Docs: Correct type for the $post parameter of the {$adjacent}_post_link filter.

The parameter was documented as `WP_Post`, but it contains the value of `get_adjacent_post()`, which returns an empty string if there is no corresponding post, so the correct type is `WP_Post|string`.

Follow-up to [11243], [16951], [28111], [32606].

Props apermo, audrasjb.
Fixes #57047.

git-svn-id: https://develop.svn.wordpress.org/trunk@54841 602fd350-edb4-49c9-b593-d223f7449a82

Author: SergeyBiryukov

src/wp-includes/link-template.php

@@ -2318,11 +2318,11 @@ function get_adjacent_post_link( $format, $link, $in_same_term = false, $exclude
 	 * @since 2.6.0
 	 * @since 4.2.0 Added the `$adjacent` parameter.
 	 *
-	 * @param string  $output   The adjacent post link.
-	 * @param string  $format   Link anchor format.
-	 * @param string  $link     Link permalink format.
-	 * @param WP_Post $post     The adjacent post.
-	 * @param string  $adjacent Whether the post is previous or next.
+	 * @param string         $output   The adjacent post link.
+	 * @param string         $format   Link anchor format.
+	 * @param string         $link     Link permalink format.
+	 * @param WP_Post|string $post     The adjacent post. Empty string if no corresponding post exists.
+	 * @param string         $adjacent Whether the post is previous or next.
 	 */
 	return apply_filters( "{$adjacent}_post_link", $output, $format, $link, $post, $adjacent );
 }