Improve Enum documentation

[dbutenhof]

#57: "The link to the Enums section is broken."

Corrected the link

#58: "Python enum data type needs explicit sub-type."

Streamlined and clarified, I think, but there's no evidence that an explicit sub-type, which essentially would change the default __str__ method, is necessary. I'm assuming that, if this is incorrect, this PR will solicit a relevant response!

resolves #58
resolves #57

---

By contributing to this repository, I agree to the contribution guidelines.

[jaredoconnell]

This is good until/if we get any more examples showing that the explicit string or int enum types are required. And there's a good chance that in those cases, it's a bug, rather than something that needs documentation changes.