We have a couple sources for in-code documentation:
  * We have docstrings on recipe module classes, module documentation, and method documentation
  * We can read module-level docstrings on a per-recipe basis
  * We can now document properties (using PROPERTIES), and can discover the full set of properties that a given recipe will respond to

We should tie this all together into a nice `recipes.py help --recipe recipe_name` and `recipes.py help --module recipe_module_name` command line interface. The first one could also work in `recipes.py run recipe_name --help` too.

The generated help text could list all valid input properties, their help strings, the docstring for the recipe itself, etc.