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

this fixes #532
There's an existing initialiser [lib/rspec/matchers/dsl.rb#L323](https://github.com/oveddan/rspec-expectations/blob/e161ca7283f0060e3245de37e4efee1db3b4dfde/lib/rspec/matchers/dsl.rb#L323)