Breaking changes docs for template index_patterns

0219a211d3 added support for templates
to have multiple patterns and renamed `template` to `index_patterns`.
This adds the breaking changes docs for that.

docs/reference/migration/migrate_6_0.asciidoc

@@ -31,6 +31,7 @@ way to reindex old indices is to use the `reindex` API.
 * <<breaking_60_cluster_changes>>
 * <<breaking_60_settings_changes>>
 * <<breaking_60_plugins_changes>>
+* <<breaking_60_indices_changes>>
 
 include::migrate_6_0/cat.asciidoc[]
 
@@ -45,3 +46,5 @@ include::migrate_6_0/cluster.asciidoc[]
 include::migrate_6_0/settings.asciidoc[]
 
 include::migrate_6_0/plugins.asciidoc[]
+
+include::migrate_6_0/indices.asciidoc[]

docs/reference/migration/migrate_6_0/indices.asciidoc

@@ -0,0 +1,29 @@
+[[breaking_60_indices_changes]]
+=== Templates changes
+
+==== `template` is now `index_patterns`
+
+Previously templates expressed the indices that they should match using a glob
+style pattern in the `template` field. They should now use the `index_patterns`
+field instead. As the name implies you can define multiple glob style patterns
+in an array but for convenience defining a single pattern as a bare string is
+also supported. So both of these examples are valid:
+
+[source,js]
+--------------------------------------------------
+PUT _template/template_1
+{
+  "index_patterns": ["te*", "bar*"],
+  "settings": {
+    "number_of_shards": 1
+  }
+}
+PUT _template/template_2
+{
+  "index_patterns": "te*",
+  "settings": {
+    "number_of_shards": 1
+  }
+}
+--------------------------------------------------
+// CONSOLE