better support for dynamic docs

[UnquietCode]

relates to #40

• remove 'dynamic documentation' return string
• add method parameter docs
  • captures either a string or a callable
  • (turn strings into callables)
• add annotation @with_docs which takes a similar arguments
  • string or callable as sole argument
  • annotation must be imported from another module, not in the immediate
    namespace
  • ensure decorated steps are still able to be located within the class

---

☀ This issue was created by email; some cleanup may be required.