Docs: Improve a comment about `$doing_wp_cron` global in `wp-cron.php`.

Props chinteshprajapati, subrataemfluence, nielslange.
Fixes #46104.
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git-svn-id: https://core.svn.wordpress.org/trunk@46533 1a063a9b-81f0-0310-95a4-ce76da25c4cd