With reference to comments #2 and #3 from this BZ https://bugzilla.redhat.com/show_bug.cgi?id=2174516 raising RFE for documentation.
https://github.com/theforeman/foreman-documentation/pull/2046
Published: - https://access.redhat.com/documentation/en-us/red_hat_satellite/6.12/html-single/managing_hosts/index#Upgrading_Hosts_to_Next_Major_RHEL_Release_managing-hosts - https://access.redhat.com/documentation/en-us/red_hat_satellite/6.11/html-single/managing_hosts/index#Upgrading_Hosts_to_Next_Major_RHEL_Release_managing-hosts - https://access.redhat.com/documentation/en-us/red_hat_satellite/6.10/html-single/managing_hosts/index#upgrading-hosts-from-rhel7-to-rhel8_managing-hosts
Documentation for Satellite 6.13 is now live at Red Hat Customer Portal: https://access.redhat.com/documentation/en-us/red_hat_satellite/6.13/html-single/managing_hosts/index?lb_target=preview&check_logged_in=1#Upgrading_Hosts_to_Next_Major_RHEL_Release_managing-hosts