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/+/200863Reviewed-by: Heschi Kreinick <heschi@google.com>

doc/articles/race_detector.html

@@ -133,6 +133,11 @@ cost of increased memory usage.
 <code>halt_on_error</code> (default <code>0</code>): Controls whether the program
 exits after reporting first data race.
 </li>
+
+<li>
+<code>atexit_sleep_ms</code> (default <code>1000</code>): Amount of milliseconds
+to sleep in the main goroutine before exiting.
+</li>
 </ul>
 
 <p>