Improve documentation for Producer header support

[andreportela]

I spent some time trying to debug this and searching the internet.

I was trying to pass headers like
headers = [("key", "value"), ...] or headers = [(b"key", b"value"), ...]

In the end, the right way to do it is headers = [("key", b"value"), ...]. I finally found it in a test on #471
Where can we document this? I would be happy to open a PR.

[tvoinarovskyi]

I would be grateful if you could. I agree we need to document this behaviour, as it's far from obvious.

[andreportela]

Anywhere in particular? Or should I just dig a little and put it where I see fit?

[tvoinarovskyi]

For one it should be described in API reference here https://aiokafka.readthedocs.io/en/stable/api.html?highlight=headers#aiokafka.AIOKafkaProducer.send. You can update it using the docstring https://github.com/aio-libs/aiokafka/blob/master/aiokafka/producer/producer.py#L364.

Maybe a few words in producer docs, as it's more a problem to send proper structure than to read what you got.