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...
Have trivialInput produce an implementer of IRichInput, whose instances will be providers of IRichInput
Update the _FiniteStateInterpreter's receive docstring
Remove 'provider' in trivialInput, fix test case name, and add newsfile
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.
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Merge pull request #15 from cyli/fix_trivial_input

Fix for trivialInput so that instances produced are providers of IRichInput
Merge pull request #22 from adamtheturtle/point-at-waffleio-21

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