HLSL: Recognize POSITION semantic et al in DX9 compatibility mode (#2255)

diff --git a/StandAlone/StandAlone.cpp b/StandAlone/StandAlone.cpp
index a7ce53d..871788e 100644
--- a/StandAlone/StandAlone.cpp
+++ b/StandAlone/StandAlone.cpp
@@ -1593,7 +1593,8 @@
            "  --hlsl-iomap                      perform IO mapping in HLSL register space\n"
            "  --hlsl-enable-16bit-types         allow 16-bit types in SPIR-V for HLSL\n"
            "  --hlsl-dx9-compatible             interprets sampler declarations as a\n"
-           "                                    texture/sampler combo like DirectX9 would.\n"
+           "                                    texture/sampler combo like DirectX9 would,\n"
+           "                                    and recognizes DirectX9-specific semantics\n"
            "  --invert-y | --iy                 invert position.Y output in vertex shader\n"
            "  --keep-uncalled | --ku            don't eliminate uncalled functions\n"
            "  --nan-clamp                       favor non-NaN operand in min, max, and clamp\n"