Make inheritance from `proto.Message` clear in docs

[busunkim96]

We've been seeing an influx of issues of folks trying to serialize/deserialize messages using the protobuf methods. I think part of the problem is that it's not clear the underlying type has changed from looking at the generated docs.

googleapis/python-vision#64
googleapis/python-vision#70
googleapis/python-memcache#19

[busunkim96]

Types for Google Cloud Datacatalog v1 API
=========================================

.. automodule:: google.cloud.datacatalog_v1.types
    :members:
    :show-inheritance:

Adding show-inheritance seems to do the trick. The remaining problem is that it is not linkified. Intersphinx seems to be failing to retrieve the objects.inv for proto-plus.

[autosummary] generating autosummary for: README.rst, UPGRADING.md, changelog.md, datacatalog_v1/services.rst, datacatalog_v1/types.rst, datacatalog_v1beta1/services.rst, datacatalog_v1beta1/types.rst, index.rst, multiprocessing.rst
loading intersphinx inventory from http://python.readthedocs.org/en/latest/objects.inv...
loading intersphinx inventory from https://google-auth.readthedocs.io/en/stable/objects.inv...
loading intersphinx inventory from https://googleapis.dev/python/google-api-core/latest/objects.inv...
loading intersphinx inventory from https://grpc.io/grpc/python/objects.inv...
loading intersphinx inventory from https://proto-plus-python.readthedocs.io/...
intersphinx inventory has moved: http://python.readthedocs.org/en/latest/objects.inv -> https://python.readthedocs.io/en/latest/objects.inv
intersphinx inventory has moved: https://grpc.io/grpc/python/objects.inv -> https://grpc.github.io/grpc/python/objects.inv
intersphinx inventory has moved: https://proto-plus-python.readthedocs.io/ -> https://proto-plus-python.readthedocs.io/en/stable/
WARNING: failed to reach any of the inventories with the following issues:
intersphinx inventory 'https://proto-plus-python.readthedocs.io/' not readable due to ValueError: unknown or unsupported inventory version: ValueError('invalid inventory header: ')