Add note about availability to printerProvider API description

extensions/common/api/printer_provider.idl

 // Copyright 2014 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
-// The <code>chrome.printerProvider</code> API exposes events used by print
+// <p>The <code>chrome.printerProvider</code> API exposes events used by print
 // manager to query printers controlled by extensions, to query their
 // capabilities and to submit print jobs to these printers.
+// <p/>
+// <p>Available only on dev channel since Chrome 42.</p>
 namespace printerProvider {
   // Error codes returned in response to $(ref:onPrintRequested) event.
   enum PrintError {
     // Operation completed successfully.
     OK,
 
     // General failure.
     FAILED,
 
     // Print ticket is invalid. For example, ticket is inconsistent with
@@ skipping 62 matching lines @@
 
     // Event fired when print manager requests printing.
     // |printJob|: The printing request parameters.
     // |resultCallback|: Callback that should be called when the printing
     // request is completed.
     static void onPrintRequested(PrintJob printJob,
                                  PrintCallback resultCallback);
   };
 };