fix doxygen brief in subzero header files

src/IceThreading.cpp

 //===- subzero/src/IceThreading.cpp - Threading function definitions ------===//
 //
 //                        The Subzero Code Generator
 //
 // This file is distributed under the University of Illinois Open Source
 // License. See LICENSE.TXT for details.
 //
 //===----------------------------------------------------------------------===//
 ///
 /// \file
-/// This file defines threading-related functions.
+/// \brief Defines threading-related functions.
 ///
 //===----------------------------------------------------------------------===//
 
 #include "IceThreading.h"
 
 #include "IceCfg.h"
 #include "IceDefs.h"
 
 namespace Ice {
 
@@ skipping 26 matching lines @@
   assert(getKind() == WI_Asm);
   return std::move(Function);
 }
 
 std::unique_ptr<Cfg> EmitterWorkItem::getCfg() {
   assert(getKind() == WI_Cfg);
   return std::move(RawFunc);
 }
 
 } // end of namespace Ice