document MitID Ervherv VAT ID prefill feature

src/pages/verify/e-ids/danish-mitid-erhverv.mdx

@@ -55,6 +55,14 @@ You can determine which kind of option the user selected by inspecting the claim
 - Company signatory logins will have a `companySignatory` claim with value `true`.
 - Private logins will have neither of the above.
 
+### Pefill VAT ID (CVR number)
+
+If you already have an expectation of what company the user will login on behalf of, you can prefill the VAT ID (CVR number)
+to limit their selection choice (if multiple roles in the same company) or skip the company selector all together.
+
+- When activating MitID Erhverv via acr_values: `urn:grn:authn:dk:mitid:business`: `login_hint=vatid:DK<CVR>`
+- When activating MitID Erhverv via login_hint: `login_hint=business vatid:DK<CVR>`
+
 ## Test users
 
 To test business logins with MitID Erhverv, you will start by [creating a test organization](/verify/e-ids/danish-mitid-erhverv/#create-test-organization) and [a test user](/verify/e-ids/danish-mitid-erhverv/#create-test-users), and then [connect the test user](/verify/e-ids/danish-mitid-erhverv/#connect-user-to-organization-as-an-employee) to the organization as an employee.