State what the phone number is used for on the checkout form

[boo-code]

Questions	Answers
Description?	The optional phone field gives no reason for asking, and buyers hesitate to hand out a phone number; checkout research recommends stating the purpose next to the field. This adds a translated hint under the delivery and invoice phone inputs — "Only used if we need to contact you about the order or its delivery." — set server-side in AddressFieldsFormatTrait through the FormField availableValues.comment slot the theme's form-fields.tpl already renders as a form-text line (no theme or JS change). Item 4 of #96.
Type?	improvement
BC breaks?	no
Deprecations?	no
Fixed ticket?
How to test?	vendor/bin/phpunit -c tests/php/phpunit.xml --filter AddressFieldsFormatTraitTest (asserts the comment on the phone field and its absence on other fields). On a 9.2 checkout the hint renders under the Phone input next to the "Optional" marker — verified live.

[ps-jarvis]

Hello @boo-code!

This is your first pull request on ps_onepagecheckout repository of the PrestaShop project.

Thank you, and welcome to this Open Source community!

[ps-jarvis]

This pull request seems to contain new translation strings. I have summarized them below to ease up review:

• Modules.Onepagecheckout.Shop ⚠️
  • Only used if we need to contact you about the order or its delivery.

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