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.

Change-Id: If199d36fd80a0945af90055d110519e5dba27361
Reviewed-on: https://go-review.googlesource.com/c/go/+/200863


Reviewed-by: Heschi Kreinick <heschi@google.com>