v10.0.0-RC2: =htc #368 document the guarantees of `max-connections` (#473)

* =htc #368 document the guarantees of `max-connections`

Actually providing better guarantees turned out to be difficult, so
being clear about the limit being an approximate one seems to be the
best solution for now.

See also the discussions in https://github.com/akka/akka-http/pull/371.

Fixes #368.

* =htc #368 be more accurate about `max-connections`

To find out whether the materialization for one connection has finished
bindAndHandle will now use two `watchTermination` combinators.

As explained in the Scaladoc this will still not provide 100 percent
guarantee that all of the flow will have finished but it is hopefully
more accurate than before.