CODE_GUIDELINES: document member_field_required and Vereinfacht required fields

CODE_GUIDELINES.md

@@ -2849,12 +2849,14 @@ Building accessible applications ensures that all users, including those with di
 
 **Required Fields:**
 
+Which member fields are required (asterisk, tooltip, validation) is configured in **Settings** (Memberdata section: edit a member field and set "Required"). The member create/edit form and Member resource validation both read `settings.member_field_required`. Email is always required; other fields default to optional.
+
 ```heex
-<!-- Mark required fields -->
+<!-- Mark required fields (value from settings or always true for email) -->
 <.input
   field={@form[:first_name]}
   label={gettext("First Name")}
-  required
+  required={@member_field_required_map[:first_name]}
   aria-required="true"
 />
 ```