fix: clarify generic OIDC provider issuer behaviour (#1642)

docs/kratos/social-signin/05_generic.mdx

@@ -109,6 +109,13 @@ mapping.
 The **Data mapping** section allows you to map the claims returned by the sign-in provider to traits as defined in the identity
 schema.
 
+:::note
+
+For generic providers with `claims_source=userinfo`, the issuer claim will be ignored if the userinfo response is not signed. This
+behavior aligns with the OpenID Connect specification.
+
+:::
+
 To define the mapping, create a Jsonnet code snippet. Read [this document](./data-mapping) to learn more about Jsonnet data
 mapping.