Add QueryJob#num_dml_affected_rows and DDL/DML docs

[quartzmo]

[closes #2141]

[blowmage]

I would like to get @tswast's review on this.

[blowmage]

LGTM, but I'd defer to @tswast.

[quartzmo]

@tswast Do you think we should add wrapper methods similar to #query for DDL and DML operations? These new methods would wait for the job to complete, then return the table reference (DDL) or the number of rows affected (DML).

[tswast]

In Python we return a job for the query helper (but then allow iterating over it for getting the results). Ideally one should be able to use the same helper to call DDL and DML queries, though it's tough when the helper returns "results".

We suffered a similar problem in Java, where my solution was to special-case an "empty results" object for DDL queries. googleapis/google-cloud-java#3469

[tswast]

Property LGTM. I think the docs to recommend the job method are good for DDL / DML, but ideally one should be able to call the regular query method if they don't care about the detailed results.

[quartzmo]

@tswast Good suggestion, I've been working today on adding DDL/DML response info to the array type returned by #query. (Which would be an empty or lazy array for these usages.)

[quartzmo]

In Python we return a job for the query helper (but then allow iterating over it for getting the results).

@tswast @blowmage PTAL at the most recent commit:

• Add DDL/DML attrs to Data, such as the table reference (DDL) or the number of rows affected (DML).
• Update #query to conditionally return empty Data populated with DDL/DML attrs.

[quartzmo]

@tswast I updated totalRows to be nil for DDL/DML, and completed docs and unit tests. I believe this PR is ready for final review.