Add Registration to Saml2Authentication

Closes gh-9487

docs/manual/src/docs/asciidoc/_includes/servlet/saml2/saml2-login.adoc

@@ -107,6 +107,7 @@ where
 * `https://idp.example.com/issuer` is the value contained in the `Issuer` attribute of the SAML responses that the identity provider will issue
 * `classpath:idp.crt` is the location on the classpath for the identity provider's certificate for verifying SAML responses, and
 * `https://idp.example.com/issuer/sso` is the endpoint where the identity provider is expecting `AuthnRequest` s.
+* `adfs` is <<servlet-saml2login-relyingpartyregistrationid, an arbitrary identifier you choose>>
 
 And that's it!
 
@@ -196,6 +197,7 @@ image:{icondir}/number_10.png[] And finally, it takes the `NameID` from the firs
 Then, it places that principal and the authorities into a `Saml2Authentication`.
 
 The resulting `Authentication#getPrincipal` is a Spring Security `Saml2AuthenticatedPrincipal` object, and `Authentication#getName` maps to the first assertion's `NameID` element.
+`Saml2AuthenticatedPrincipal#getRelyingPartyRegistrationId` holds the <<servlet-saml2login-relyingpartyregistrationid,identifier to the associated `RelyingPartyRegistration`>>.
 
 [[servlet-saml2login-opensaml-customization]]
 ==== Customizing OpenSAML Configuration
@@ -410,6 +412,10 @@ open fun relyingPartyRegistrations(): RelyingPartyRegistrationRepository? {
 ----
 ====
 
+[[servlet-saml2login-relyingpartyregistrationid]]
+[NOTE]
+The `registrationId` is an arbitrary value that you choose for differentiating between registrations.
+
 Or you can provide each detail manually, as you can see below:
 
 .Relying Party Registration Repository Manual Configuration