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

this fixes #532
> As far as the configuration of this value, the natural inclination would be for it to be in RSpec::Expectations::Configuration since its value is consumed in this library.

Correct.  That's where the config option belongs.

> But if rspec-core is generating spec_helper, and is setting a default value for this, shouldn't the config for this be located in the Core configuration, even though it's not consumed in that library? That feels a bit weird, right?

I don't think it feels weird.  We already have lines in the generated `spec_helper` that set rspec-expectations and rspec-mocks config settings.