Poll async jobs using correct state logic in BQ usage docs.

[tseaver]

Closes: #2119.

[dhermes]

LGTM. Does the state come back from the server? Should we normalize to ensure it is always uppercase (or lowercase)?

[tseaver]

Does the state come back from the server?

Yes, it is read-only.

Should we normalize to ensure it is always uppercase (or lowercase)?

Likely not. We could look at adding convenience properties, e.g. job.pending, job.running, job.done, which would mask the string literals.