clarify the default setting of --track-allocation option

[bicycle1885]

Because I forget things easily. The default value is set here:

julia/src/jloptions.c

Lines 472 to 487 in 34f7a4a

	case opt_track_allocation:
	if (optarg != NULL) {
	if (!strcmp(optarg,"user"))
	malloclog = JL_LOG_USER;
	else if (!strcmp(optarg,"all"))
	malloclog = JL_LOG_ALL;
	else if (!strcmp(optarg,"none"))
	malloclog = JL_LOG_NONE;
	else
	jl_errorf("julia: invalid argument to --track-allocation (%s)", optarg);
	break;
	}
	else {
	malloclog = JL_LOG_USER;
	}
	break;

.