Updates to Upgrade Kibana docs for 6.x+ (#13897)

docs/setup/upgrade.asciidoc

@@ -27,25 +27,11 @@ should follow:
 |4.x >= 4.2     |5.x            |<<upgrade-standard,Standard upgrade>>
 |5.0.0 pre GA   |5.x            |<<upgrade-standard-reindex,Standard upgrade & reindex>>
 |5.x            |5.y            |<<upgrade-standard,Standard upgrade>> (where `y > x`)
+|5.x            |6.x            |<<upgrade-standard,Standard upgrade>> & <<migrating-6.0-index,Reindex transformation>>
 |=======================================================================
 
-[IMPORTANT]
-===========================================
-Reindex is required if you ever upgraded from Kibana 4.1 or 4.0
-
-Even if you're running Kibana version 4.2 or higher, if you have previously
-upgraded from Kibana 4.0 or 4.1, you will need to do a
-<<upgrade-standard-reindex,Standard upgrade & reindex>> when upgrading to 5.x.
-
-This is due to the Elasticsearch requirement that any index that was initially
-created before Elasticsearch 2.0 must be reindexed even if it's currently
-working in Elasticsearch 2.x.
-
-You only need to do this once when moving to 5.x.
-===========================================
-
-NOTE: Saved searches, visualizations, and dashboards created in Kibana 4.x are
-generally compatible with 5.x.
+NOTE: Saved searches, visualizations, and dashboards created in Kibana 4.x or 5.x
+can be generally imported into 6.x.
 
 include::upgrade/upgrade-standard.asciidoc[]