Docs: Document the $wp_meta_keys global in meta registration functions.

Follow-up to [37924].

Props noruzzaman, mukesh27.
See #64224.

git-svn-id: https://develop.svn.wordpress.org/trunk@61584 602fd350-edb4-49c9-b593-d223f7449a82

Author: SergeyBiryukov

src/wp-includes/meta.php

@@ -1397,6 +1397,8 @@ function sanitize_meta( $meta_key, $meta_value, $object_type, $object_subtype =
  * @since 6.4.0 The `$revisions_enabled` argument was added to the arguments array.
  * @since 6.7.0 The `label` argument was added to the arguments array.
  *
+ * @global array $wp_meta_keys Global registry for meta keys.
+ *
  * @param string       $object_type Type of object metadata is for. Accepts 'blog', 'post', 'comment', 'term',
  *                                  'user', or any other object type with an associated meta table.
  * @param string       $meta_key    Meta key to register.
@@ -1567,6 +1569,8 @@ function register_meta( $object_type, $meta_key, $args, $deprecated = null ) {
  *
  * @since 5.5.0
  *
+ * @global array $wp_meta_keys Global registry for meta keys.
+ *
  * @param mixed  $value     Current value passed to filter.
  * @param int    $object_id ID of the object metadata is for.
  * @param string $meta_key  Metadata key.
@@ -1646,6 +1650,8 @@ function registered_meta_key_exists( $object_type, $meta_key, $object_subtype =
  * @since 4.6.0
  * @since 4.9.8 The `$object_subtype` parameter was added.
  *
+ * @global array $wp_meta_keys Global registry for meta keys.
+ *
  * @param string $object_type    Type of object metadata is for. Accepts 'blog', 'post', 'comment', 'term',
  *                               'user', or any other object type with an associated meta table.
  * @param string $meta_key       Metadata key.
@@ -1696,6 +1702,8 @@ function unregister_meta_key( $object_type, $meta_key, $object_subtype = '' ) {
  * @since 4.6.0
  * @since 4.9.8 The `$object_subtype` parameter was added.
  *
+ * @global array $wp_meta_keys Global registry for meta keys.
+ *
  * @param string $object_type    Type of object metadata is for. Accepts 'blog', 'post', 'comment', 'term',
  *                               'user', or any other object type with an associated meta table.
  * @param string $object_subtype Optional. The subtype of the object type. Default empty string.