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

this fixes #532
* Would be nice if the reader method had a question mark to make it clear it's a boolean predicate.
* The YARD docs don't mention that it should be a Boolean -- would be nice if they would.  Currently the rendered docs say this attribute is an object.
* I think that "chained expectations" is a confusing phrase -- I would consider `expect(x).to matcher_1.and_matcher_2.and matcher_3` to be "chained expectations" (as this chains together multiple expectations).  Instead, I'd phrase this like:

``` ruby
# Sets or gets if custom matcher descriptions and failure messages
# should include clauses from methods defined using `chain`. It is
# false by default for backwards compatibility.
```