Enhance documentation about absl::Duration integer division, which

truncates, and floating-point multiplication/division, which rounds.

PiperOrigin-RevId: 517016481
Change-Id: I3b0c357ea7236f2c0231df5e6b47c742c3f40047

absl/time/time.h

@@ -187,7 +187,12 @@ class Duration {
   Duration& operator%=(Duration rhs);
 
   // Overloads that forward to either the int64_t or double overloads above.
-  // Integer operands must be representable as int64_t.
+  // Integer operands must be representable as int64_t. Integer division is
+  // truncating, so values less than the resolution will be returned as zero.
+  // Floating-point multiplication and division is rounding (halfway cases
+  // rounding away from zero), so values less than the resolution may be
+  // returned as either the resolution or zero.  In particular, `d / 2.0`
+  // can produce `d` when it is the resolution and "even".
   template <typename T, time_internal::EnableIfIntegral<T> = 0>
   Duration& operator*=(T r) {
     int64_t x = r;