Update Python docs to with correct timestamp granularity

[rnpridgeon]

Description

Python documentation incorrectly states that the timestamp field should use microseconds. If we take a look at rdkafka.h however we can see that the expected unit is actually Milliseconds

Python Docs
https://docs.confluent.io/current/clients/confluent-kafka-python/#confluent_kafka.Producer.produce

rdkafka.h
https://github.com/edenhill/librdkafka/blob/13befa7053c43593fd6f2effdffbd83c3ea52eb7/src/rdkafka.h#L804

This will ultimately cause issues from Kafka as the broker will actually record this timestamp which will break things

https://issues.apache.org/jira/browse/KAFKA-6939

How to reproduce

produce a message with a timestamp with microsecond granularity.

Checklist

Please provide the following information:

• confluent-kafka-python and librdkafka version (confluent_kafka.version() and confluent_kafka.libversion()):
• Apache Kafka broker version:
• Client configuration: {...}
• Operating system:
• Provide client logs (with 'debug': '..' as necessary)
• Provide broker log excerpts
• Critical issue

[johnistan]

Let me tell you pain this can cause!!!!

[edenhill]

@johnistan Sorry about that :( such a small but important mistake

[qiao-meng-zefr]

Great find. We've had issues with this too. It's better to just leave it out for default after all..