docs/stdlib/stream: correct comment in example code about when data loss

In the example code the condition `done & self.stream.valid & ~self.stream.ready`
is not sufficient to cause the payload to be discarded. That is because the done
bit remains high. In case of a very slow stream, the first two quickly arriving
serial transactions will never be lost, even if the first transaction has not
been processed yet, when the second one arrives.
However if a 3rd transaction arrives as well, then the 2nd transaction will be
discarded.

docs/stdlib/stream.rst

@@ -121,7 +121,9 @@ In this example, the external device does not provide a way to pause data transm
                 m.d.sync += done.eq(0)
             with m.Elif(self.stream.ready):
                 m.d.sync += self.stream.valid.eq(0)
-            # Payload is discarded if `done & self.stream.valid & ~self.stream.ready`.
+            # Pending payload in `data` is discarded if:
+            # `done & self.stream.valid & ~self.stream.ready`,
+            # and a new serial transaction begins
 
             return m