Clarify compatibility in upgrading stack document

When doing upgrade testing from 5.6.14 to 6.6.0 using this doc:
https://www.elastic.co/guide/en/elastic-stack/6.6/upgrading-elastic-stack.html

When ES/Kibana are upgraded to 6.6.0 and Logstash is still on 5.6.14. I was confused by the below statement, when I went to check monitoring for Logstash and it did not appear.

Beats and Logstash 5.6 are compatible with Elasticsearch 6.6.0 to give you flexibility in scheduling the upgrade

@ycombinator pointed me to this matrix to verify monitoring 5.6 is not compatible with 6.6.0

https://www.elastic.co/support/matrix#matrix_compatibility

Maybe we can clarify a bit in the upgrading elastic stack document too about that.