Self-managed SAML Group Sync Documentation

The following discussion from !76390 (merged) should be addressed:

• @eread started a discussion: (+3 comments)

  @dblessing I guess the docs for this will be similar to having: https://docs.gitlab.com/ee/administration/auth/ldap/ldap_synchronization.html#group-sync, but on the https://docs.gitlab.com/ee/integration/saml.html page?