[release-branch.go1] doc: Don't imply incorrect guarantees about data races.

««« backport dbcfb4459ab8
doc: Don't imply incorrect guarantees about data races.

A race between
        a = "hello, world"
and
        print(a)
is not guaranteed to print either "hello, world" or "".
Its behaviour is undefined.

Fixes #4039.

R=rsc
CC=dvyukov, gobot, golang-dev, r
https://golang.org/cl/6489075

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