[Documentation] Support for streaming

[vincent-163]

I was surprised to find out that mysql2 actually has a streaming interface compatible with mysql, but only without promises. I didn't figure out how to use it in the promises API, and both are not documented. Will there be documentation for this? It would be even more helpful if an async iterator is implemented for promises API.

[sidorares]

Some discussion started in #677 ( I feel there was a more detailed discussion somewhere but can't find it, will try to link later )

[sidorares]

Here is it: #822 (comment)