Added link to the interactive upgrade guide & tweaked the intro.

Author: debadair

docs/upgrading-stack.asciidoc

@@ -1,11 +1,32 @@
 [[upgrading-elastic-stack]]
 == Upgrading the Elastic Stack
 
-We recommend upgrading your stack to 5.6 and using the {xpack} Upgrade Assistant
-to prepare for your upgrade to {version}. You can perform a rolling upgrade when
-upgrading from 5.6 to 6.x, a full cluster restart is generally not required.
-Upgrading from any other version than 5.6 requires a full cluster
-restart.
+When upgrading to a new version of Elasticsearch, you need to upgrade
+each of the products in your Elastic Stack. Beats and Logstash 5.6 are
+compatible with Elasticsearch 6.0 to give you flexibility in scheduling the
+upgrade.
+
+****
+The steps you need to take to upgrade differ depending on which products you
+are using. Want a list that's tailored to your stack? Try out our
+{upgrade_guide}[Interactive Upgrade Guide].
+****
+
+We recommend upgrading to 5.6 before upgrading to {version}. {xpack} 5.6
+provides a free Upgrade Assistant that identifies issues you need to address
+before upgrading and simplifies migrating indices that need to be reindexed
+before you upgrade. The Upgrade Assistant is enabled with both Trial and
+Basic licenses. You can install X-Pack solely for the purpose of upgrading.
+There's no obligation to continue using X-Pack once your upgrade is complete.
+
+Rolling upgrades are supported when upgrading from Elasticsearch 5.6 to
+{version}, a full cluster restart is not always required. Upgrading from any
+other version than 5.6 requires a full cluster restart.
+
+IMPORTANT: 2.x indices are not compatible with {version}. You must
+remove or reindex them on 5.x 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}.
 
 === Preparing to upgrade