Documentation: comments in index templates (#20701)

Fixes #20513

docs/reference/indices/templates.asciidoc

@@ -40,6 +40,9 @@ PUT _template/template_1
 // CONSOLE
 // TESTSETUP
 
+NOTE: Index templates provide C-style /* */ block comments. Comments are allowed
+everywhere in the JSON document except before to the initial opening curly bracket. 
+
 Defines a template named template_1, with a template pattern of `te*`.
 The settings and mappings will be applied to any index name that matches
 the `te*` template.