PPAPI: Clean up documentation for InstanceHandle and PASS_REF

ppapi/cpp/pass_ref.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 PPAPI_CPP_PASS_REF_H_
 #define PPAPI_CPP_PASS_REF_H_
 
+/// @file
+/// This file defines an annotation for constructors and other functions that
+/// take ownership of a pointer.
 namespace pp {
 
 /// An annotation for constructors and other functions that take ownership of
 /// a pointer. For example, a resource constructor that takes ownership of a
-/// passed in PP_Resource ref count would take this to differentiate from
-/// the more typical use case of taking its own ref.
+/// provided <code>PP_Resource</code> ref count would take this enumeration to
+/// differentiate from the more typical use case of taking its own reference.
 enum PassRef { PASS_REF };
 
 }  // namespace pp
 
 #endif  // PPAPI_CPP_PASS_REF_H_