runtime: specify behavior of SetMutexProfileFraction for negative values

Change-Id: Ie4da1a515d5405140d742bdcd55f54a73a7f71fe
Reviewed-on: https://go-review.googlesource.com/108175Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/runtime/mprof.go

@@ -434,7 +434,7 @@ var mutexprofilerate uint64 // fraction sampled
 // reported. The previous rate is returned.
 //
 // To turn off profiling entirely, pass rate 0.
-// To just read the current rate, pass rate -1.
+// To just read the current rate, pass rate < 0.
 // (For n>1 the details of sampling may change.)
 func SetMutexProfileFraction(rate int) int {
 	if rate < 0 {