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

this fixes #532
> I think that making it configurable per matcher will be very annoying. Among other things, that means there's no way for us to default it to enabled for new projects like we can do with a config option in spec_helper. Users aren't likely to even know about the extra `include_chained_clauses` method to think to call it.

Either way they'll have to know about it if they have a mix of matchers.

> You mention 3rd party matchers but in my experience libraries of matchers tend to be implemented from scratch using a class rather than using the DSL.

I've come across several cases of clients using private gems to share such things and they're more likely to be built using the DSL in that case (similar to how we share spec support stuff from `rspec-support`, they're built for internal use only so never quite as polished).