Document there is no backpressure on HTTP2

[josevalim]

Relates to #282.

[hissssst]

I think that it is also worth documenting for async_stream too. And overall approach must be pull model over push model, that means that async_stream needs to be reworked too

[josevalim]

async_request already documents that it is a firehose. I think it is pretty much meant to be a fire and forget. If you want back-pressure, then I think pointing to one of the stream functions is fine.

[sneako]

Thank you!