Fail start on obsolete indices documentation

Improved documentation on when nodes will refuse to start up.

Issue elastic#27073

Author: henningandersen

docs/reference/migration/migrate_7_0.asciidoc

@@ -26,7 +26,7 @@ See also <<release-highlights>> and <<es-release-notes>>.
 * <<breaking_70_restclient_changes>>
 * <<breaking_70_low_level_restclient_changes>>
 * <<breaking_70_logging_changes>>
-* <<breaking_70_node_start>>
+* <<breaking_70_node_changes>>
 
 [float]
 === Indices created before 7.0
@@ -61,4 +61,4 @@ include::migrate_7_0/snapshotstats.asciidoc[]
 include::migrate_7_0/restclient.asciidoc[]
 include::migrate_7_0/low_level_restclient.asciidoc[]
 include::migrate_7_0/logging.asciidoc[]
-include::migrate_7_0/node_start.asciidoc[]
+include::migrate_7_0/node.asciidoc[]

docs/reference/migration/migrate_7_0/node.asciidoc

@@ -0,0 +1,15 @@
+[float]
+[[breaking_70_node_changes]]
+=== Node start up
+
+[float]
+==== Nodes with left-behind data or metadata refuse to start
+Repurposing an existing node by changing node.master or node.data to false can leave lingering on-disk metadata and
+data around, which will not be accessible by the node's new role. Beside storing non-accessible data, this can lead
+to situations where dangling indices are imported even though the node might not be able to host any shards, leading
+to a red cluster health. To avoid this,
+
+* nodes with on-disk shard data and node.data set to false will refuse to start
+* nodes with on-disk index/shard data and both node.master and node.data set to false will refuse to start
+
+Beware that such role changes done prior to the 7.0 upgrade could prevent node start up in 7.0.