docs: add priority_delivery shipment option documentation

[Piet-Hein]

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[FreekVR]

The description for "mailbox package" earlier in chapter 6.A.1 needs updating as well, as it mentions you should not use any shipment options. It should reference priority_delivery here, or we should just remove all these warnings about "additional" options?

6.A.1 

This package type is available on [platform 1 (myparcel)](https://deploy-preview-193--myparcel-developer-portal.netlify.app/api-reference/04.data-types.html#platform) and [platform 2 (flespakket)](https://deploy-preview-193--myparcel-developer-portal.netlify.app/api-reference/04.data-types.html#platform) for PostNL NL shipments that fit into a mailbox. It does not support additional options. On [platform 3 (belgie)](https://deploy-preview-193--myparcel-developer-portal.netlify.app/api-reference/04.data-types.html#platform) this package type is available for bpost BE shipments that fit into a mailbox. It does not support additional options. This package type is also available on [platform 1 (myparcel)](https://deploy-preview-193--myparcel-developer-portal.netlify.app/api-reference/04.data-types.html#platform) for international shipments that fit into a mailbox. Shipment option tracked is available. You need a PostNL contract for international mailbox packages if you want to use this product for your shipments. If you don't have such a contract, you can't create international mailbox package shipments.

If you still make the request with additional options, bear in mind that you may pay more than is necessary!

Additionally, 6.A.3 mentions shipment options are exclusive to platform 1 / package type package. This is already incorrect as shipment options are also available on other platforms AND for package small (and now mailbox). Suggest removing this part.

These are the different package options such as insurance, recipient only, signature on receipt, XL format etc. These options are specified in the [ShipmentOptions](https://deploy-preview-193--myparcel-developer-portal.netlify.app/api-reference/07.shipment-related-object-definitions.html#_7-c) object. These options are only available for [package-type 1 (package)](https://deploy-preview-193--myparcel-developer-portal.netlify.app/api-reference/04.data-types.html#package-type).