docs: note client potentially ignoring connection reset

Encountered this while adding the server message queue, where
a server exception caused the client to raise ConnectionAbortedError in
AsyncClient.send_message() > _drain() > _writer.drain()
and yet the client was still attempting to read.

src/dumdum/client/async_client.py

@@ -130,6 +130,8 @@ async def _read_loop(
         writer: asyncio.StreamWriter,
     ) -> None:
         while True:
+            # FIXME: may not terminate if drain() from another task results
+            #        in connection reset
             data = await reader.read(1024)
             if len(data) == 0:
                 break