Document http server shutdown semantics in low level docs

**Issue by [ktoso](https://github.com/ktoso)**
_Tuesday Mar 29, 2016 at 22:30 GMT_
_Originally opened as https://github.com/akka/akka/issues/20177_

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We should have an explicit section about shutting down semantics.
Remember that we have graceful shutdown (doesn't kill connections) by default and by design currently.
The other one is `shutdownNow` which we do not have nowadays but can be emulated with KillSwitch-es.

Locate the low-level HTTP server documentation and review its existing shutdown guidance first. Document that graceful shutdown is the default and does not kill connections, and explain that shutdownNow is unavailable but can be emulated with KillSwitches.

Written by the indexing model from the issue text.