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

this fixes #532
> The decision to include it was influenced by "reset_syntaxes_to_default" - is that used anywhere besides the tests?

It's used here:

https://github.com/rspec/rspec-expectations/blob/775f931a2f57100cf38118392e8c49ea01c7e033/lib/rspec/expectations/configuration.rb#L133

Also, the "default" for the syntaxes is more involved because we issue a deprecation warning if users use the old `:should` syntax without explicitly configuring it -- so this method helps manage that.