improve documentation for -HC switches

compiler/src/dmd/cli.d

@@ -417,22 +417,21 @@ dmd -cov -unittest myprog.d
  Option("Hf=<filename>",
  "write 'header' file to filename"
  ),
- Option("HC[=[silent|verbose]]",
- "generate C++ 'header' file",
- `Generate C++ 'header' files using the given configuration:",
+ Option("HC[=[?|h|help|silent|verbose]]",
+ "write C++ 'header' equivalent to stdout",
+ `write C++ 'header' equivalent to stdout configured with:",
  $(DL
+ $(DT ?|h|help)$(DD list available options for C++ 'header' file generation)
  $(DT silent)$(DD only list extern(C[++]) declarations (default))
  $(DT verbose)$(DD also add comments for ignored declarations (e.g. extern(D)))
  )`,
  ),
- Option("HC=[?|h|help]",
- "list available modes for C++ 'header' file generation"
- ),
  Option("HCd=<directory>",
- "write C++ 'header' file to directory"
+ "write C++ 'header' file to directory",
+ "write C++ 'header' file to directory, ignored if -HCf=<filename> is not present",
  ),
  Option("HCf=<filename>",
- "write C++ 'header' file to filename"
+ "write C++ 'header' file to filename instead of stdout"
  ),
  Option("-help",
  "print help and exit"