Docs for preventing migrations failing for unknown saved object types

docs/setup/upgrade/upgrade-migrations.asciidoc

@@ -58,6 +58,29 @@ complete the upgrade migration before bringing up the remaining instances.
 ==== Preventing migration failures
 Review the common causes of {kib} upgrade failures and how to prevent them.
 
+[float]
+====== Documents were found for unknown saved object types
+Migrations will fail if saved objects were found which belong to an unknown
+saved object type. This could be caused by writing malformed documents directly
+to the Elasticsearch index, or by disabling a plugin that had previously
+created a saved object.
+
+We recommend using the <<upgrade-assistant,Upgrade Assistant>> to discover and
+remedy any unknown saved object types. In addition to the upgrade assistant, 
+{kib} version 7.17.0 deployments containing unknown saved object types will 
+also log the following warning message:
+
+[source,sh]
+--------------------------------------------
+CHECK_UNKNOWN_DOCUMENTS Upgrades will fail for 8.0+ because documents were found for unknown saved object types. To ensure that upgrades will succeed in the future, either re-enable plugins or delete these documents from the ".kibana_7.17.0_001" index after the current upgrade completes.
+--------------------------------------------
+
+If you fail to remedy this, your upgrade to 8.0+ will fail with a message like:
+[source,sh]
+--------------------------------------------
+Unable to complete saved object migrations for the [.kibana] index: Migration failed because documents were found for unknown saved object types. To proceed with the migration, please delete these documents from the ".kibana_7.17.0_001" index.
+--------------------------------------------
+
 [float]
 ===== timeout_exception or receive_timeout_transport_exception
 There is a known issue in 7.12.0 for users who tried the {fleet} beta.