fix doxygen brief in subzero header files

src/IceAssemblerX8664.h

 //===- subzero/src/IceAssemblerX8664.h - Assembler for x86-64 ---*- C++ -*-===//
 //
 // Copyright (c) 2013, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 //
 // Modified by the Subzero authors.
 //
 //===----------------------------------------------------------------------===//
 //
 //                        The Subzero Code Generator
 //
 // This file is distributed under the University of Illinois Open Source
 // License. See LICENSE.TXT for details.
 //
 //===----------------------------------------------------------------------===//
 ///
 /// \file
-/// This file implements the Assembler class for x86-64.
+/// \brief Implements the Assembler class for x86-64.
 ///
 //===----------------------------------------------------------------------===//
 
 #ifndef SUBZERO_SRC_ICEASSEMBLERX8664_H
 #define SUBZERO_SRC_ICEASSEMBLERX8664_H
 
 #include "IceAssembler.h"
 #include "IceAssemblerX86Base.h"
 #include "IceDefs.h"
 #include "IceOperand.h"
@@ skipping 22 matching lines @@
 
   static bool classof(const Assembler *Asm) {
     return Asm->getKind() == Asm_X8664;
   }
 };
 
 } // end of namespace X8664
 } // end of namespace Ice
 
 #endif // SUBZERO_SRC_ICEASSEMBLERX8664_H