Review and clarify legal warnings in tax details modal

[haszari]

Describe the bug

In the modal for adding merchant tax details, there is a warning/requirement:

If your sales exceed the VAT threshold for your country, you're required to register for a VAT Number.

When working on adding Japan support, @haszari removed this warning (for Japan only), since it mentions "VAT" directly (Japan does not call it VAT), and also it's unclear what the rules are in Japan.

If we want to warn merchants about the legalities of registering / providing tax info, we should do this in documentation, and link to it from the UI. There's likely a lot of detail/nuance that would clutter the dialog (and is slow to update).

A new merchant seeing this warning might ask:

• What are the thresholds?
• How do I register for a VAT number?

Ideally we would guide them to relevant info.

The goal of this issue is to either remove this legal warning, or reword it, and consider adding a link to docs, e.g. add some info to Tax Documents page (or link out to expertise).

To Reproduce

1. Set up tax docs so you have a document to view at Payments > Documents. Fake one up with this CLI command:
   • wp wcpay add_document_for_account [account_id] vat_invoice --mode=test --period_from="2024-04-01 00:00:00" --period_to="2024-04-30 23:59:59"
   • Or can use wp wcpay backfill_merchant_vat_invoices
2. If you've added VAT id / tax information previously, clear it out so you can see the modal:
   • Delete company_vat_submitted company_vat_number company_name company_address from wcpay_account_meta table.
   • Side note:
     • https://github.com/Automattic/woocommerce-payments-server/issues/5915
3. Go to Payments > Documents.
4. Click Download for a tax doc row.
5. See modal.

Actual behavior

Confusing warning about legality/regulations in specific regions.

Expected behavior

Soften warning, and link out to docs for accurate details in each region.