Docs: add documentation about removal of cluster.routing.allocation.snapshot.relocation_enabled

docs/reference/migration/migrate_6_0.asciidoc

@@ -28,6 +28,7 @@ way to reindex old indices is to use the `reindex` API.
 * <<breaking_60_search_changes>>
 * <<breaking_60_docs_changes>>
 * <<breaking_60_cluster_changes>>
+* <<breaking_60_settings_changes>>
 * <<breaking_60_plugins_changes>>
 
 include::migrate_6_0/rest.asciidoc[]
@@ -38,4 +39,6 @@ include::migrate_6_0/docs.asciidoc[]
 
 include::migrate_6_0/cluster.asciidoc[]
 
+include::migrate_6_0/settings.asciidoc[]
+
 include::migrate_6_0/plugins.asciidoc[]

docs/reference/migration/migrate_6_0/settings.asciidoc

@@ -0,0 +1,10 @@
+[[breaking_60_settings_changes]]
+=== Settings changes
+
+==== Snapshot settings
+
+The internal setting `cluster.routing.allocation.snapshot.relocation_enabled` that allowed shards with running snapshots to be reallocated to
+different nodes has been removed. Enabling this setting could cause allocation issues if a shard got allocated off a node and then
+reallocated back to this node while a snapshot was running.
+
+