NameID mapping and Single Logout (elastic#47288)

Clarify in the documentation that for SAML Single Logout to be
functional, the Identity Provider needs to release a NameID.

Author: jkakavas

x-pack/docs/en/security/authentication/saml-guide.asciidoc

@@ -394,6 +394,7 @@ services it offers.
 By default the Elastic Stack will support SAML SLO if the following are true:
 
 - Your IdP metadata specifies that the IdP offers a SLO service
+- Your IdP releases a NameID in the subject of the SAML assertion that it issues for your users
 - You configure `sp.logout`
 - The setting `idp.use_single_logout` is not `false`