document why keys aren't settable in typed producer

[cyberhck]

while #82 will allow us to set keys to a message through middleware, we don't allow this by design in typed producer.

in 99% of the cases, we're not supposed to use keys, it's supposed to be managed by kafka.
in the odd times when we do need that functionality we should either use a middleware or raw producer.

The initial KP release was such brutal that it was not possible to set key at all, but we needed the functionality to have keys and realized we can't avoid 100% of the time. I'll write this in docs later.