Docs: Add a file header to wp-includes/class-wp-meta-query.php, introduced in [33761].

Also clarifies the class DocBlock summary for `WP_Meta_Query`.

See #33413. See #33701.


git-svn-id: https://develop.svn.wordpress.org/trunk@34405 602fd350-edb4-49c9-b593-d223f7449a82

src/wp-includes/class-wp-meta-query.php

@@ -1,6 +1,16 @@
 <?php
 /**
- * Class for generating SQL clauses that filter a primary query according to metadata keys and values.
+ * Meta API: WP_Meta_Query class
+ *
+ * @package WordPress
+ * @subpackage Meta
+ * @since 4.4.0
+ */
+
+/**
+ * Core class used to implement meta queries for the Meta API.
+ *
+ * Used for generating SQL clauses that filter a primary query according to metadata keys and values.
  *
  * `WP_Meta_Query` is a helper that allows primary query classes, such as {@see WP_Query} and {@see WP_User_Query},
  * to filter their results by object metadata, by generating `JOIN` and `WHERE` subclauses to be attached