Clarify docs a bit.
Use a new config instance for these specs.

- Makes the `it "is false by default"` spec more accurate (as a new
  instance demonstrates the default more accurately then manually
  setting it to `nil`).
- Prevents the changes here from "leaking" into other specs;
  previously, the mutation to the global config could affect
  other specs.
Set the `include_chain_clauses...` config globally.

This is more desirable behavior and will be the
default in RSpec 4.
Add changelog for #600.
Merge pull request #609 from rspec/pr-600-fixups

Pr 600 followups