BigQuery usage docs do not explain references

The BigQuery usage docs do not explain what TableReferences are, when/why you’d need to use one instead of a Table, and how to get a TableReference from a Table or vice versa.

(And similarly for DatasetReferences and Datasets.)

[tswast]

Re: my previous comment.

I’ve sent https://github.com/GoogleCloudPlatform/google-cloud-python/pull/5255 to add Dataset/Table.from_string(fully_qualified_id), which I think will address the concern that it requires too many objects to create a table/dataset.

[tswast]

Honestly since the REST API separates everything out like

{"projectId": "my-project", "datasetId": "my_dataset"}

it hadn’t crossed my mind to accept a fully-qualified dataset ID. I would be open to a PR that modifies Dataset to accept strings like “project.dataset” as an option where there is a Dataset reference.