Document the ACMRandom::Rand31() method.

Change-Id: I09fda19ff9c156e6075fda1bcd5c02391d1ba1c8
diff --git a/test/acm_random.h b/test/acm_random.h
index 0a8317f..a14b671 100644
--- a/test/acm_random.h
+++ b/test/acm_random.h
@@ -26,6 +26,7 @@
 
   void Reset(int seed) { random_.Reseed(seed); }
 
+  // Generates a random 31-bit unsigned integer from [0, 2^31).
   uint32_t Rand31(void) {
     return random_.Generate(testing::internal::Random::kMaxRange);
   }