aioclient: More accurate DFU docstring

pytec/pytec/aioclient.py

@@ -243,11 +243,11 @@ class AsyncioClient:
         await self.disconnect()
 
     async def dfu(self):
-        """Put the Thermostat in DFU update mode
+        """Put the Thermostat in DFU mode
 
         The client is disconnected as the Thermostat stops responding to
-        TCP commands in DFU update mode. The only way to exit it is by
-        power-cycling.
+        TCP commands in DFU mode. To exit it, submit a DFU leave request
+        or power-cycle the Thermostat.
         """
         async with self._command_lock:
             self._writer.write("dfu\n".encode("utf-8"))