Add missing API documentation for `IOutputExecutor.output` and fix the `output_` method signatures in the turnstile example.
Add a news fragment for this fix.

Hopefully this will feed into Twisted's `NewsBuilder` and make releases easier.
Worst case I'll manually concatenate a bunch of files...
Simplify.  I'll add real documentation for rich inputs real soon now, promise.
Merge pull request #11 from hybridcluster/rich-input-context-docs-9

Fix the `output_` signatures in the example to match what is actually required by machinist.