Add an explanatory comment to the test.

src/libraries/System.Security.Cryptography/tests/HashAlgorithmTest.cs

@@ -66,6 +66,11 @@ public async Task ComputeHashAsync_SupportsCancellation()
             using (PositionValueStream stream = new SelfCancellingStream(10000, cancellationSource))
             using (HashAlgorithm hash = new SummingTestHashAlgorithm())
             {
+                // The stream has a length longer than ComputeHashAsync's read buffer,
+                // so ReadAsync will get called multiple times.
+                // The first call succeeds, but moves the cancellation source to canceled,
+                // and the second call then fails with an OperationCanceledException, cancelling the
+                // whole operation.
                 await Assert.ThrowsAnyAsync<OperationCanceledException>(
                     () => hash.ComputeHashAsync(stream, cancellationSource.Token));