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.

Read the upgrading Elastic Stack document linked in the issue, then compare its compatibility wording with the referenced support matrix. Clarify the statement so the documented upgrade flexibility and the monitoring compatibility concern are not conflated; done means the document accurately explains the relevant compatibility limits.

Written by the indexing model from the issue text.