Improve documentation for Matchers module

[pbrisbin]

The examples in the ViaAWS documentation are good, but not enough for someone completely unfamiliar to understand what's going on. Particularly how Matchers enable targeting specific AWS requests and dictating the response that will be returned in the derived MonadAWS instance.

I think header docs on the Matcher module itself, linked from the MockAWS module docs, would be a good place to explain this concept.