doc/articles/race_detector: mention memory leak potential

[sthagen]

As far as I can tell, there is no public documentation on this topic,
which cost me several days of debugging.

I am possibly unusual in that I run binaries in production with the
race detector turned on, but I think that others who do the same may
want to be aware of the risk.

Updates golang#26813.
Updates golang#37233.

Change-Id: I1f8111bd01d0000596e6057b7cb5ed017d5dc655
Reviewed-on: https://go-review.googlesource.com/c/go/+/220586
Reviewed-by: Dmitri Shuralyov dmitshur@golang.org

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