Add note about availability to printerProvider API description

extensions/common/api/printer_provider.idl

Index: extensions/common/api/printer_provider.idl
diff --git a/extensions/common/api/printer_provider.idl b/extensions/common/api/printer_provider.idl
index f730addfb63291fe00e22ba4c85183b89d49b1c1..940e018e58d54a91dc116b544062a059b61f07a5 100644
--- a/extensions/common/api/printer_provider.idl
+++ b/extensions/common/api/printer_provider.idl
@@ -2,9 +2,11 @@
 // 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 {