fix doxygen brief in subzero header files

src/IceTargetLowering.cpp

 //===- subzero/src/IceTargetLowering.cpp - Basic lowering implementation --===//
 //
 //                        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 skeleton of the TargetLowering class, specifically
-/// invoking the appropriate lowering method for a given instruction kind and
-/// driving global register allocation. It also implements the non-deleted
-/// instruction iteration in LoweringContext.
+/// \brief Implements the skeleton of the TargetLowering class.
+///
+/// Specifically this invokes the appropriate lowering method for a given
+/// instruction kind and driving global register allocation. It also implements
+/// the non-deleted instruction iteration in LoweringContext.
 ///
 //===----------------------------------------------------------------------===//
 
 #include "IceTargetLowering.h"
 
 #include "IceAssemblerARM32.h"
 #include "IceAssemblerMIPS32.h"
 #include "IceAssemblerX8632.h"
 #include "IceAssemblerX8664.h"
 #include "IceCfg.h" // setError()
@@ skipping 639 matching lines @@
   if (Target == Target_##X)                                                    \
     return TargetHeader##X::create(Ctx);
 #include "llvm/Config/SZTargets.def"
 
   llvm::report_fatal_error("Unsupported target header lowering");
 }
 
 TargetHeaderLowering::~TargetHeaderLowering() = default;
 
 } // end of namespace Ice