document that HTTP route construction must not close over Actor state

**Issue by [rkuhn](https://github.com/rkuhn)**
_Thursday Feb 12, 2015 at 20:01 GMT_
_Originally opened as https://github.com/akka/akka/issues/16871_

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The pattern we show should be inspired by the Props recommendation: build the Route in a function within the companion object to make sure that nothing is pulled in accidentally, all captured values are explicit.

Locate the HTTP route construction examples and the existing Props recommendation in the documentation. Document the companion-object pattern, making clear that route construction must not close over Actor state and that captured values should be explicit. Done means the guidance is present alongside the relevant route example.

Written by the indexing model from the issue text.