Add a couple of comments clarifying `dot_` and `FuncOutput`

PiperOrigin-RevId: 508019650
diff --git a/hwy/examples/benchmark.cc b/hwy/examples/benchmark.cc
index 55afd3b..14a4929 100644
--- a/hwy/examples/benchmark.cc
+++ b/hwy/examples/benchmark.cc
@@ -144,7 +144,10 @@
     sum0 = Add(sum0, sum1);
     sum2 = Add(sum2, sum3);
     sum0 = Add(sum0, sum2);
+    // Remember to store the result in `dot_` for verification; see `Verify`.
     dot_ = GetLane(SumOfLanes(d, sum0));
+    // Return the result so that the benchmarking framework can ensure that the
+    // computation is not elided by the compiler.
     return static_cast<FuncOutput>(dot_);
   }
   void Verify(size_t num_items) {