Update the Vercel documentation

[joren485]

Description

Domain takeovers using Vercel are definitely still possible.

However, they are limited. In my testing, I found that a domain is not vulnerable if:

• The root domain is used by a Vercel account (i.e. the root domain points to 76.76.21.21 and is linked to a project).
• The domain/root domain is verified, even if the root domain does not point to 76.76.21.21.
• Another subdomain of the same root domain is used by a Vercel account.

This PR updates the Vercel documentation to reflect this.

[EdOverflow]

Thank you, @joren485!

[zangcc]

Is this vulnerability no longer exploitable? Why hasn't the Status changed to Not vulnerable?

@EdOverflow @joren485