message: Docs: update -E documentation
diff: @@ -672,20 +672,24 @@ To generate a text assembly file, pass ``--emit-asm``:
  ::
  - ispc foo.ispc -o foo.asm --emit-asm
  + ispc foo.ispc -o foo.s --emit-asm
  To generate LLVM bitcode, use the ``--emit-llvm`` flag.
  To generate LLVM bitcode in textual form, use the ``--emit-llvm-text`` flag.
  -To generate a stub after running the preprocessor, use the ``-E`` flag.
  +To run only the preprocessor, use the ``-E`` flag.
  ::
  - ispc foo.ispc -E -o foo.i OR ispc foo.ispc -E -o foo.ispi
  + ispc foo.ispc -E -o foo.i
  + ispc foo.ispc -E -o foo.ispi
  -In this mode, the preprocessor will assume ``stdout`` if no output file is
  -specified.
  -Both ``.i`` or ``.ispi`` are valid suffixes.
  +In this mode, the output will be directed to ``stdout`` if no output file is
  +specified. The standard suffixes ``.i`` or ``.ispi`` are assumed for preprocessor output.
  +
  +By default the compilation will fail if preprocessor encountered an error.
  +To ignore the preprocessor errors and proceed with normal compilation flow,
  +``--ignore-preprocessor-errors`` switch may be used.
  Optimizations are on by default; they can be turned off with ``-O0``:
  