Spring Security SAML: Document how to change validation for audiences when using example IssuerSaml2AuthenticationRequestFactory

Summary

The documentation shows as example IssuerSaml2AuthenticationRequestFactory on how to get "more control", but it does not show, how to update the validation for SAML2AssertionValidationParameters.COND_VALID_AUDIENCES when the IdP uses the "calculated" issuer as audience.

https://github.com/spring-projects/spring-security/blob/master/docs/manual/src/docs/asciidoc/_includes/servlet/saml2/saml2-login.adoc

Expected Behavior

Updated documentation.