I always have to look up which are the thread-safe ones. So add this
documentation in counters.h.

R=mstarzinger@chromium.org

Bug: v8:8834
Change-Id: I2cd28b6604585152b1ba3ecacbd597166f1b0e5b
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1520716
Commit-Queue: Michael Starzinger <mstarzinger@chromium.org>
Auto-Submit: Clemens Hammacher <clemensh@chromium.org>
Reviewed-by: Michael Starzinger <mstarzinger@chromium.org>
Cr-Commit-Position: refs/heads/master@{#60214}