[spirv] Fix doc about half translation (#1539)

docs/SPIR-V.rst

@@ -566,9 +566,9 @@ type instructions:
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 Please note that ``half`` is translated into 32-bit floating point numbers
-right now because MSDN says that "this data type is provided only for language
-compatibility. Direct3D 10 shader targets map all ``half`` data types to
-``float`` data types."
+if without ``-enable-16bit-types`` because MSDN says that "this data type
+is provided only for language compatibility. Direct3D 10 shader targets map
+all ``half`` data types to ``float`` data types."
 
 Minimal precision scalar types
 ------------------------------