TWO-24757/docs: Document the deliberate self-invoice upload gap

[dgjlindsay]

WooCommerce will not get merchant self-invoice upload (TWO-24757 cancelled). Unlike PrestaShop — which renders its native invoice PDF and uploads it on fulfilment — WooCommerce has no native invoice renderer, so the feature would mean bundling a PDF generator (dompdf) into client ecommerce sites. That is a maintenance and security overhead we are not taking on without merchant demand.

This PR adds a comment in on_order_completed marking exactly where the implementation goes (Action Scheduler job after the fulfillments call) if demand materialises; TWO-24757 retains the full design.

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Author ✍️@dgjlindsay