Query: Some documentation and test improvements for update_post_author_caches():

* Make the descriptions for `update_post_author_caches()` and `update_post_caches()` more specific.
* Move the unit test into its own file, for consistency with `update_post_cache()` tests. This also allows for using shared fixtures in case more tests are added in the future.

Follow-up to [53482].

See #55716.

git-svn-id: https://develop.svn.wordpress.org/trunk@53483 602fd350-edb4-49c9-b593-d223f7449a82

tests/phpunit/tests/post/updatePostCache.php

@@ -1,12 +1,12 @@
 <?php
 /**
- * Test `wp_update_cache()`.
+ * Test `update_post_cache()`.
  *
  * @package WordPress
  */
 
 /**
- * Test class for `wp_update_cache()`.
+ * Test class for `update_post_cache()`.
  *
  * @group post
  * @group query
@@ -18,7 +18,7 @@ class Tests_Post_UpdatePostCache extends WP_UnitTestCase {
 	/**
 	 * Post IDs from the shared fixture.
 	 *
-	 * @var array
+	 * @var int[]
 	 */
 	public static $post_ids;