Document remote monitoring cluster version requirement for upgrades?

We have a note here on version dependency, i.e. they can't do an export from a 6.x cluster to a 5.x monitoring cluster during upgrades: https://www.elastic.co/guide/en/x-pack/current/monitoring-production.html#monitoring-production

The problem is that this note is in the "setup" section of the monitoring guide and users are unlikely going to revisit this page as part of the upgrade. It will be helpful if we can call this out (or cross link) as part of the stack upgrade documentation.