doc: document atexit_sleep_ms flag in race_detector
It was pointed out to me that the thread sanitizer library has many more
flags than the ones documented in this doc page. In particular, I was
able to make use of GORACE=atexit_sleep_ms=10 to reduce the amount that
'go test -race' will sleep before finishing.
I'm sure that this flag will be useful to others in the future, so add
it here. This is still not a complete list, but we can simply add useful
flags that work when we think they deserve a mention.