Clarify purpose of handler_stream_test.go (#472)

Clarify that the sole purpose of the test in `handler_stream_test.go` is
to verify a safety property of the `ClientStream` iterator. A similar
testing strategy isn't worthwhile for `ServerStream` or `BidiStream`,
because they're not iterators.

---------

Co-authored-by: Akshay Shah <akshay@akshayshah.org>

handler_stream_test.go

@@ -22,14 +22,17 @@ import (
  pingv1 "connectrpc.com/connect/internal/gen/connect/ping/v1"
 )
 
-func TestClientStream(t *testing.T) {
+func TestClientStreamIterator(t *testing.T) {
  t.Parallel()
+ // The server's view of a client streaming RPC is an iterator. For safety,
+ // and to match grpc-go's behavior, we should allocate a new message for each
+ // iteration.
  stream := &ClientStream[pingv1.PingRequest]{conn: &nopStreamingHandlerConn{}}
  assert.True(t, stream.Receive())
  first := fmt.Sprintf("%p", stream.Msg())
  assert.True(t, stream.Receive())
  second := fmt.Sprintf("%p", stream.Msg())
- assert.NotEqual(t, first, second)
+ assert.NotEqual(t, first, second, assert.Sprintf("should allocate a new message for each iteration"))
 }
 
 type nopStreamingHandlerConn struct {