dbus: Add comments about the right way to expose methods

dbus/bus.h

 // Copyright (c) 2012 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.
 
 #ifndef DBUS_BUS_H_
 #define DBUS_BUS_H_
 
 #include <dbus/dbus.h>
 
 #include <map>
@@ skipping 403 matching lines @@
   // or failed to be obtained, in the origin thread.
   //
   // Must be called in the origin thread.
   virtual void RequestOwnership(const std::string& service_name,
                                 ServiceOwnershipOptions options,
                                 OnOwnershipCallback on_ownership_callback);
 
   // Requests the ownership of the given service name.
   // Returns true on success, or the the service name is already obtained.
   //
+  // Note that it's important to expose methods before requesting a service
+  // name with this method.  See also ExportedObject::ExportMethodAndBlock()
+  // for details.
+  //
   // BLOCKING CALL.
   virtual bool RequestOwnershipAndBlock(const std::string& service_name,
                                         ServiceOwnershipOptions options);
 
   // Releases the ownership of the given service name.
   // Returns true on success.
   //
   // BLOCKING CALL.
   virtual bool ReleaseOwnership(const std::string& service_name);
 
@@ skipping 310 matching lines @@
 
   std::string address_;
   base::Closure on_disconnected_closure_;
 
   DISALLOW_COPY_AND_ASSIGN(Bus);
 };
 
 }  // namespace dbus
 
 #endif  // DBUS_BUS_H_