Clarify the difference between packaging text and packaging input

[teolemon]

What

• Add text to clarify the difference between packaging text and packaging input. Mention that packaging text will be taken into account into computations, but that the preferred method is structured input

Mockup

Part of

• 6374

[github-actions[bot]]

This issue has been open 90 days with no activity. Can you give it a little love by linking it to a parent issue, adding relevant labels and projets, creating a mockup if applicable, adding code pointers from https://github.com/openfoodfacts/openfoodfacts-server/blob/main/.github/labeler.yml, giving it a priority, editing the original issue to have a more comprehensive description… Thank you very much for your contribution to 🍊 Open Food Facts