missing docs for enums: switch to module-based reference docs for enums module

[tswast]

There are several enums in https://github.com/googleapis/python-bigquery/blob/master/google/cloud/bigquery/enums.py which are undocumented.

I recall we were doing class-based docs mostly because the "jobs" module was too huge to properly browse. It should be safe to use module-based docs for the smaller modules like enums, so we don't have to keep remembering to keep reference.rst in sync.