Emphasise singleRequest more in docs

**Issue by [ktoso](https://github.com/ktoso)**
_Monday Mar 14, 2016 at 22:20 GMT_
_Originally opened as https://github.com/akka/akka/issues/20030_

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As it's the simplest and most likely the API most people should be using.
Currently docs come in order from "lowest level to" "request level", which noone understands that that is the "simple one".

Start by locating the documentation that describes the request APIs and reviewing how singleRequest is currently presented relative to lower-level and request-level APIs. Done means the docs clearly emphasize singleRequest as the simplest, recommended API and make its intended use obvious.

Written by the indexing model from the issue text.