fix: add clarification

Author: vinckr

docs/kratos/manage-identities/15_customize-identity-schema.mdx

@@ -1313,7 +1313,10 @@ You may also mix usernames and passwords:
 ### Advanced schema
 
 The following identity schema defines personal fields for first name, last name, nickname, and age, and includes two checkboxes:
-one required for accepting the terms of service and one optional for newsletter subscription.
+one required for accepting the terms of service and one optional for newsletter subscription.  
+To render links in a checkbox label, use markdown link syntax. Links open in a new tab.  
+To make a checkbox required, add `"const": true` to the property definition. This differs from text fields, which use the required
+array.
 
 ````mdx-code-block
 <Tabs>