Add to documentation for GetIdentityHash

include/v8.h

 // Copyright 2007-2008 the V8 project authors. All rights reserved.
 // Redistribution and use in source and binary forms, with or without
 // modification, are permitted provided that the following conditions are
 // met:
 //
 //     * Redistributions of source code must retain the above copyright
 //       notice, this list of conditions and the following disclaimer.
 //     * Redistributions in binary form must reproduce the above
 //       copyright notice, this list of conditions and the following
 //       disclaimer in the documentation and/or other materials provided
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   /**
    * Turns on access check on the object if the object is an instance of
    * a template that has access check callbacks. If an object has no
    * access check info, the object cannot be accessed by anyone.
    */
   void TurnOnAccessCheck();
 
   /**
    * Returns the identity hash for this object. The current implemenation uses
-   * a hidden property on the object to store the identity hash.
+   * a hidden property on the object to store the identity hash. 
+   *
+   * The return value will never be 0. Also, it is not guaranteed to be 
+   * unique.
    */
   int GetIdentityHash();
 
   /**
    * Access hidden properties on JavaScript objects. These properties are
    * hidden from the executing JavaScript and only accessible through the V8
    * C++ API. Hidden properties introduced by V8 internally (for example the
    * identity hash) are prefixed with "v8::".
    */
   bool SetHiddenValue(Handle<String> key, Handle<Value> value);
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 }  // namespace v8
 
 
 #undef V8EXPORT
 #undef V8EXPORT_INLINE
 #undef TYPE_CHECK
 
 
 #endif  // V8_H_