docs: specify the inherited-members directive for job classes (#2244)

It seems that versions of python earlier than 3.10 may have had issues
processing inherited members annotations, and accidentally include
inherited members by default.

As we recently worked to excise older versions of python in this repo,
it seems we're now correctly processing sphinx directives, which means
we no longer emit docstrings for inherited members.

This PR adds a minor sphinx directive to include inherited members
for the job classes, and I've confirmed locally by running the `docsfx`
nox job that the inherited members do now get included in the docfx_yaml
output.

Author: shollyman

docs/reference.rst

@@ -22,6 +22,7 @@ Job
 ===
 
 .. automodule:: google.cloud.bigquery.job
+   :inherited-members:
 
 .. toctree::
   :maxdepth: 2