Add documentation of Shibboleth repository requirement to reference docs

[mminella]

The documentation for the newer requirement to obtain OpenSAML dependencies is in release notes for a specific version (from what I can find) and not referenced in the reference documentation. Does it make sense to add a small snip in the reference docs so that it is easier to find?

Related:
#1584
https://docs.spring.io/spring-security/reference/servlet/saml2/login/overview.html#servlet-saml2login-minimaldependencies