Docs: Add missing `@global` reference for the `$post_id` global in `wp_admin_bar_edit_menu()`.

Follow-up to [47903].

Props davidbaumwald.
See #50312.
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git-svn-id: https://core.svn.wordpress.org/trunk@47680 1a063a9b-81f0-0310-95a4-ce76da25c4cd