Update documentation to reflection disuse of external cvtres.

lld/COFF/Driver.cpp

void LinkerDriver::link(ArrayRef<const char *> ArgsArr) {
 
   // We should have inferred a machine type by now from the input files, but if
   // not we assume x64.
   if (Config->Machine == IMAGE_FILE_MACHINE_UNKNOWN) {
     warn("/machine is not specified. x64 is assumed");
     Config->Machine = AMD64;
   }
 
-  // Windows specific -- Input files can be Windows resource files (.res files).
-  // We invoke cvtres.exe to convert resource files to a regular COFF file
-  // then link the result file normally.
+  // Input files can be Windows resource files (.res files). We use
+  // WindowsResource to convert resource files to a regular COFF file,
+  // then link the resulting file normally.
   if (!Resources.empty())
     addBuffer(convertResToCOFF(Resources));
 
   if (Tar)
     Tar->append("response.txt",
                 createResponseFile(Args, FilePaths,
                                    ArrayRef<StringRef>(SearchPaths).slice(1)));
 

lld/docs/windows_support.rst

Creating DLL
   :good:`Done`. LLD creates a DLL if ``/DLL`` option is given. Exported
   functions can be specified either via command line (``/EXPORT``) or via
   module-definition file (.def). Both export-by-name and export-by-ordinal are
   supported. LLD uses Microsoft ``lib.exe`` tool to create an import library
   file.
 
 Windows resource files support
   :good:`Done`. If an ``.res`` file is given, LLD converts the file to a COFF
-  file using ``cvtres.exe`` command and link it.
+  file using LLVM's Object library.
 
 Safe Structured Exception Handler (SEH)
   :good:`Done` for both x86 and x64.
 
 Module-definition file
   :partial:`Partially done`. LLD currently recognizes these directives:
   ``EXPORTS``, ``HEAPSIZE``, ``STACKSIZE``, ``NAME``, and ``VERSION``.