Add doc + _AVAILABLE for simd/x86.h

include/boost/predef/hardware/simd/x86.h

@@ -9,20 +9,43 @@ http://www.boost.org/LICENSE_1_0.txt)
 #ifndef BOOST_PREDEF_HARDWARE_SIMD_X86_H
 #define BOOST_PREDEF_HARDWARE_SIMD_X86_H
 
-// from the newest to the oldest
+/*`
+ [heading `BOOST_HW_SIMD_X86`]
+
+ The SIMD extension for x86 (if detected).
+ Version number depends on the detected extension.
+
+ BOOST_HW_SIMD_X86 is defined by the first detected extension. They are
+ checked from the newest to the oldest. (AVX2 -> AVX -> ... -> SSE2 -> SSE)
+
+ [table
+     [[__predef_symbol__] [__predef_version__]]
+
+     [[`BOOST_HW_SIMD_X86`] [__predef_detection__]]
+     [[`BOOST_HW_SIMD_X86`] [V.R.P]]
+     ]
+ */
+
+// From the newest to the oldest
 #include <boost/predef/hardware/simd/x86/mic.h>
 #include <boost/predef/hardware/simd/x86/avx2.h>
-#include <boost/predef/hardware/simd/x86/fma3.h>
-#include <boost/predef/hardware/simd/x86/xop.h>
-#include <boost/predef/hardware/simd/x86/fma4.h>
 #include <boost/predef/hardware/simd/x86/avx.h>
 #include <boost/predef/hardware/simd/x86/sse4_2.h>
 #include <boost/predef/hardware/simd/x86/sse4_1.h>
 #include <boost/predef/hardware/simd/x86/ssse3.h>
-#include <boost/predef/hardware/simd/x86/sse4a.h>
 #include <boost/predef/hardware/simd/x86/sse3.h>
 #include <boost/predef/hardware/simd/x86/sse2.h>
 #include <boost/predef/hardware/simd/x86/sse.h>
 #include <boost/predef/hardware/simd/x86/mmx.h>
 
+// Those ones are more like options than real extensions
+#include <boost/predef/hardware/simd/x86/sse4a.h>
+#include <boost/predef/hardware/simd/x86/fma3.h>
+#include <boost/predef/hardware/simd/x86/fma4.h>
+#include <boost/predef/hardware/simd/x86/xop.h>
+
+#if defined(BOOST_HW_SIMD_X86)
+#   define BOOST_HW_SIMD_X86_AVAILABLE
+#endif
+
 #endif