syscall: document that Exec wraps execve(2)

Change-Id: I611511434f37c75f77c22f61f469108243bc0101
Reviewed-on: https://go-review.googlesource.com/29121Reviewed-by: Russ Cox <rsc@golang.org>

src/syscall/exec_unix.go

@@ -241,7 +241,7 @@ func StartProcess(argv0 string, argv []string, attr *ProcAttr) (pid int, handle
 	return pid, 0, err
 }
 
-// Ordinary exec.
+// Exec invokes the execve(2) system call.
 func Exec(argv0 string, argv []string, envv []string) (err error) {
 	argv0p, err := BytePtrFromString(argv0)
 	if err != nil {