Docstrings do not specify which fields are one-ofs

[busunkim96]

It is not currently possibly to tell that a field is part of a one-of by just looking at the docstring.

Example:

https://github.com/googleapis/python-dlp/blob/4f3148e93ec3dfc9395aa38a3afc62498500a055/google/cloud/dlp_v2/types/dlp.py#L3740-L3761

class Action(proto.Message):
    r"""A task to execute on the completion of a job.
    See https://cloud.google.com/dlp/docs/concepts-actions to learn
    more.
    Attributes:
        save_findings (~.dlp.Action.SaveFindings):
            Save resulting findings in a provided
            location.
        pub_sub (~.dlp.Action.PublishToPubSub):
            Publish a notification to a pubsub topic.
        publish_summary_to_cscc (~.dlp.Action.PublishSummaryToCscc):
            Publish summary to Cloud Security Command
            Center (Alpha).
        publish_findings_to_cloud_data_catalog (~.dlp.Action.PublishFindingsToCloudDataCatalog):
            Publish findings to Cloud Datahub.
        job_notification_emails (~.dlp.Action.JobNotificationEmails):
            Enable email notification for project owners
            and editors on job's completion/failure.
        publish_to_stackdriver (~.dlp.Action.PublishToStackdriver):
            Enable Stackdriver metric dlp.googleapis.com/finding_count.
    """

All of the above fields are part of a one-of 'action'.

https://github.com/googleapis/python-dlp/blob/4f3148e93ec3dfc9395aa38a3afc62498500a055/google/cloud/dlp_v2/types/dlp.py#L3837-L3862

    save_findings = proto.Field(
        proto.MESSAGE, number=1, oneof="action", message=SaveFindings,
    )


    pub_sub = proto.Field(
        proto.MESSAGE, number=2, oneof="action", message=PublishToPubSub,
    )


    publish_summary_to_cscc = proto.Field(
        proto.MESSAGE, number=3, oneof="action", message=PublishSummaryToCscc,
    )


    publish_findings_to_cloud_data_catalog = proto.Field(
        proto.MESSAGE,
        number=5,
        oneof="action",
        message=PublishFindingsToCloudDataCatalog,
    )


    job_notification_emails = proto.Field(
        proto.MESSAGE, number=8, oneof="action", message=JobNotificationEmails,
    )


    publish_to_stackdriver = proto.Field(
        proto.MESSAGE, number=9, oneof="action", message=PublishToStackdriver,
    )

This can lead to confusing behavior when the developer tries to (1) set multiple fields in the constructor or (2) sets multiple fields in multiple calls. In both cases only the last field is preserved.

Follow up to googleapis/proto-plus-python#170