Allow Creating RelyingPartyRegistration from Metadata InputStream

Update SAML2 Login reference documentation to reflect the changes

Closes gh-9558

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

@@ -536,7 +536,6 @@ RelyingPartyRegistration relyingPartyRegistration = RelyingPartyRegistrations
         .registrationId("my-id")
         .build();
 ----
-
 .Kotlin
 [source,kotlin,role="secondary"]
 ----
@@ -547,6 +546,20 @@ val relyingPartyRegistration = RelyingPartyRegistrations
 ----
 ====
 
+Note that you can also create a `RelyingPartyRegistration` from an arbitrary `InputStream` source.
+One such example is when the metadata is stored in a database:
+
+[source,java]
+----
+String xml = fromDatabase();
+try (InputStream source = new ByteArrayInputStream(xml.getBytes())) {
+    RelyingPartyRegistration relyingPartyRegistration = RelyingPartyRegistrations
+            .fromMetadata(source)
+            .registrationId("my-id")
+            .build();
+}
+----
+
 Though a more sophisticated setup is also possible, like so:
 
 ====