[7.x] [DOCS] Update version detail in upgrade instructions (#221)

Author: lcawl

docs/en/install-upgrade/upgrading-stack.asciidoc

@@ -23,8 +23,8 @@ Rolling upgrades are supported when upgrading from Elasticsearch 5.6 and
 Elasticsearch 6.0-6.2 to {version}. Upgrading from any
 version prior to 5.6 requires a full cluster restart.
 
-IMPORTANT: 2.x indices are not compatible with {version}. You must
-remove or reindex them on your 5.n cluster before upgrading to {version}. The internal
+IMPORTANT: 5.x and older indices are not compatible with {version}. You must
+remove or reindex them on your 6.x cluster before upgrading to {version}. The internal
 Kibana and {xpack} indices and the default Beats and Logstash mapping templates
 also need to be updated to work with {version}.