Add docs for saml:AuthnContextClassRef filter

modules/saml/docs/authproc_authncontextclassref.md

+`saml:AuthnContextClassRef`
+===========================
+
+IDP-side filter for setting the `AuthnContextClassRef` element in the authentication response.
+
+Examples
+--------
+
+    'authproc.idp' => array(
+      92 => array(
+        'class' => 'saml:AuthnContextClassRef',
+        'AuthnContextClassRef' => 'urn:oasis:names:tc:SAML:2.0:ac:classes:PasswordProtectedTransport',
+      ),
+    ),