Added context to custom attribute requirements

ClaimTypes referencing an extension attribute start with `extension_` but the reader is never informed to why that is. Now they will be.

Author: bolt-io

articles/active-directory-b2c/user-flow-custom-attributes.md

@@ -143,9 +143,12 @@ You can create these attributes by using the portal UI before or after you use t
 
 |Name     |Used in |
 |---------|---------|
-|`extension_loyaltyId`  | Custom policy|
+|`extension_loyaltyId` | Custom policy|
 |`extension_<b2c-extensions-app-guid>_loyaltyId`  | [Microsoft Graph API](microsoft-graph-operations.md#application-extension-directory-extension-properties)|
 
+> [!NOTE] 
+> When using custom attributes in custom policies, the claim type ID must be prefixed with `extension_` to allow the correct data mapping to take place within the B2C directory.
+
 The following example demonstrates the use of custom attributes in an Azure AD B2C custom policy claim definition.
 
 ```xml