tsan: add a comment to CallUserSignalHandler

compiler-rt/lib/tsan/rtl/tsan_interceptors_posix.cpp

static void CallUserSignalHandler(ThreadState *thr, bool sync, bool acquire,
   const int saved_errno = errno;
   errno = 99;
   // This code races with sigaction. Be careful to not read sa_sigaction twice.
   // Also need to remember pc for reporting before the call,
   // because the handler can reset it.
   volatile uptr pc = (sigactions[sig].sa_flags & SA_SIGINFO)
                          ? (uptr)sigactions[sig].sigaction
                          : (uptr)sigactions[sig].handler;
-  if (pc != sig_dfl && pc != sig_ign)
+  if (pc != sig_dfl && pc != sig_ign) {
+    // The callback can be either sa_handler or sa_sigaction.
+    // They have different signatures, but we assume that passing
+    // additional arguments to sa_handler works and is harmless.
     ((__sanitizer_sigactionhandler_ptr)pc)(sig, info, uctx);
+  }
   if (!ctx->after_multithreaded_fork) {
     thr->ignore_reads_and_writes = ignore_reads_and_writes;
     if (ignore_reads_and_writes)
       thr->fast_state.SetIgnoreBit();
     thr->ignore_interceptors = ignore_interceptors;
     thr->ignore_sync = ignore_sync;
     thr->in_symbolizer = in_symbolizer;
   }