Fail start on obsolete indices documentation

Added breaking changes documentation for node start up obsolete indices
detection.

Issue elastic#27073

Author: henningandersen

docs/reference/migration/migrate_7_0.asciidoc

@@ -25,6 +25,7 @@ See also <<release-highlights>> and <<es-release-notes>>.
 * <<breaking_70_snapshotstats_changes>>
 * <<breaking_70_restclient_changes>>
 * <<breaking_70_low_level_restclient_changes>>
+* <<breaking_70_node_start>>
 
 [float]
 === Indices created before 7.0
@@ -58,3 +59,4 @@ include::migrate_7_0/scripting.asciidoc[]
 include::migrate_7_0/snapshotstats.asciidoc[]
 include::migrate_7_0/restclient.asciidoc[]
 include::migrate_7_0/low_level_restclient.asciidoc[]
+include::migrate_7_0/node_start.asciidoc[]

docs/reference/migration/migrate_7_0/node_start.asciidoc

@@ -0,0 +1,19 @@
+[float]
+[[breaking_70_node_start]]
+=== Node start up
+
+[float]
+==== Node start up detects and fails on obsolete indices
+To avoid importing obsolete dangling indices, node start up will fail in following two situations:
+
+* Starting up a non-data node that has shard data (e.g. dedicated master node or coordinating-only node)
+* Starting up a non-data, non-master node that has index metadata.
+
+This situation should only occur if the settings node.data or node.master have been changed. Following are the primary
+situations to be aware of:
+
+* A node previously configured as node.data=true and node.master=true is changed to node.data=false
+* A node previously configured as node.data=false and node.master=true is changed to node.master=false
+* A node previously configured as node.data=true and node.master=false  is changed to node.data=false and node.master=true
+
+Beware that such changes done prior to the 7.0 upgrade could prevent starting nodes.