Docs: Improve various globals documentation, as per documentation standards.

Props upadalavipul.
See #57069, #56792.

Built from https://develop.svn.wordpress.org/trunk@54953


git-svn-id: http://core.svn.wordpress.org/trunk@54505 1a063a9b-81f0-0310-95a4-ce76da25c4cd

wp-includes/option.php

@@ -1033,6 +1033,8 @@ function set_transient( $transient, $value, $expiration = 0 ) {
  * The multi-table delete syntax is used to delete the transient record
  * from table a, and the corresponding transient_timeout record from table b.
  *
+ * @global wpdb $wpdb WordPress database abstraction object.
+ *
  * @since 4.9.0
  *
  * @param bool $force_db Optional. Force cleanup to run against the database even when an external object cache is used.