[counters] Slighly extend documentation

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}

src/counters.h

@@ -1269,6 +1269,7 @@ class RuntimeCallTimerScope {
      0, 1024, 64)
 
 #define HISTOGRAM_TIMER_LIST(HT)                                               \
+  /* Timer histograms, not thread safe: HT(name, caption, max, unit) */        \
   /* Garbage collection timers. */                                             \
   HT(gc_context, V8.GCContext, 10000,                                          \
      MILLISECOND) /* GC context cleanup time */                                \
@@ -1300,6 +1301,7 @@ class RuntimeCallTimerScope {
      MICROSECOND)
 
 #define TIMED_HISTOGRAM_LIST(HT)                                               \
+  /* Timer histograms, thread safe: HT(name, caption, max, unit) */            \
   /* Garbage collection timers. */                                             \
   HT(gc_compactor, V8.GCCompactor, 10000, MILLISECOND)                         \
   HT(gc_compactor_background, V8.GCCompactorBackground, 10000, MILLISECOND)    \