enum classes are missing docstrings

[tswast]

googleapis/python-bigquery#278 (comment)

Ideally, there would be a docstring for each enum showing all the available values. This is a regression from the monolith generator.

[software-dov]

Just curious, but is a docstring necessary for autogenerated documentation that's derived from the docstrings? Because it's easy to just...look at the values.

[tswast]

Docstring isn't 100% necessary, but it would be good to show the values in the docs themselve. Having to click "view source" is a regression.

[busunkim96]

Looks like adding undoc-members to types.rst fixes this. Change pending in #714.