Clarify default OAuth 2.0 login page requirements

Fixes gh-5996

Author: vpavic

docs/manual/src/docs/asciidoc/_includes/servlet/additional-topics/oauth2.adoc

@@ -96,6 +96,10 @@ The following sections go into more detail on each of the configuration options
 By default, the OAuth 2.0 Login Page is auto-generated by the `DefaultLoginPageGeneratingFilter`.
 The default login page shows each configured OAuth Client with its `ClientRegistration.clientName` as a link, which is capable of initiating the Authorization Request (or OAuth 2.0 Login).
 
+[NOTE]
+In order for `DefaultLoginPageGeneratingFilter` to show links for configured OAuth Clients, the registered `ClientRegistrationRepository` needs to also implement `Iterable<ClientRegistration>`.
+See `InMemoryClientRegistrationRepository` for reference.
+
 The link's destination for each OAuth Client defaults to the following:
 
 `OAuth2AuthorizationRequestRedirectFilter.DEFAULT_AUTHORIZATION_REQUEST_BASE_URI` + "/{registrationId}"