Fix incomplete documentation for AIOKafkaConsumer.offset_for_times (a…

…io-libs#1068)

Updates the return description to be in line with the behaviour of
the offset_for_times function and the official Kafka documentation.

aiokafka/consumer/consumer.py

@@ -927,7 +927,8 @@ async def offsets_for_times(self, timestamps):
         Returns:
             dict(TopicPartition, OffsetAndTimestamp): mapping from
             partition to the timestamp and offset of the first message with
-            timestamp greater than or equal to the target timestamp.
+            timestamp greater than or equal to the target timestamp. None will
+            be returned for the partition if there is no such message.
 
         Raises:
             ValueError: If the target timestamp is negative