runtime, runtime/race: add missing if(raceenabled), update package docs of pkg race

R=dvyukov
CC=golang-dev
https://golang.org/cl/6733058

src/pkg/runtime/malloc.goc

@@ -715,6 +715,7 @@ runtime·cnew(Type *typ)
 	uint32 flag;
 	void *ret;
 
+	if(raceenabled)
 		m->racepc = runtime·getcallerpc(&typ);
 	flag = typ->kind&KindNoPointers ? FlagNoPointers : 0;
 	ret = runtime·mallocgc(typ->size, flag, 1, 1);

src/pkg/runtime/race/race.go

@@ -4,7 +4,7 @@
 
 // +build race,linux,amd64 race,darwin,amd64
 
-// Data race detection.
+// Package race provides low-level facilities for data race detection.
 package race
 
 /*