Document the purpose of VIES validation and disable it by default

Problem/Motivation

According to the documentation of the European Union, there are multiple possible reasons the VIES service can return an invalid status for a VAT number:

• the VAT number does not exist
• the VAT number has not been activated for intra-EU transactions
• the registration is not yet finalised (some EU countries require a separate registration for intra-EU transactions).

This makes it so valid VAT numbers can fail validation. I feel like this is unexpected and should be better documented.

I would even go further and disable the validation by default. This module claims to validate VAT numbers, but the VIES number validates a specific aspect of VAT numbers that's probably not relevant for most users, so it shouldn't be enabled by default.

Also, VIES is not a database but a search engine. We should probably change the wording.

Proposed resolution

Improve documentation about the purpose of the VIES search engine and disable it by default.