add chained expectation descriptions to the default descriptions of custom matchers with fluent interfaces

this fixes #532
> Do you think this is a likely case? If so, and the value is configured, the libraries with the out of date rspec versions would raise an error since they would not be aware of that configuration method.

Sure...that's just like any other new API that we add in a minor release.  If gems choose to use the new API, consuming projects have to be on or above the release that added the API.  That's the way this always works.