Add comment to _wbinvd function to avoid -Wdocumentation error

Author: hdoc

clang/lib/Headers/ia32intrin.h

@@ -533,8 +533,13 @@ __rdtscp(unsigned int *__A) {
 /// \see __rdpmc
 #define _rdpmc(A) __rdpmc(A)
 
-;
-
+/// Invalidates the contents of the processor's internal caches.
+///    This function writes back and invalidates all modified cache lines in
+///    the processor.
+///
+/// \headerfile <x86intrin.h>
+///
+/// This intrinsic corresponds to the \c WBINVD instruction.
 static __inline__ void __DEFAULT_FN_ATTRS
 _wbinvd(void) {
   __builtin_ia32_wbinvd();