Add a comment that out_indent is part of formatter_out_functions since ocaml 4.06.0

stdlib/format.mli

@@ -822,6 +822,7 @@ type formatter_out_functions = {
 - the [out_indent] function performs new line indentation when the
   pretty-printer splits the line. It is called with the indentation value of
   the new line.
+  @since 4.06.0
 
   By default:
 - fields [out_string] and [out_flush] are output device specific;